diff --git a/position.rb b/position.rb
new file mode 100644
index 0000000..abb70f7
--- /dev/null
+++ b/position.rb
@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+class Position
+  # マスを'f3','d6'などの表記で表現する。変数名cell_refとして取り扱う。
+  COL = %w[a b c d e f g h].freeze
+  ROW = %w[1 2 3 4 5 6 7 8].freeze
+
+  DIRECTIONS = [
+    TOP_LEFT      = :top_left,
+    TOP           = :top,
+    TOP_RIGHT     = :top_right,
+    LEFT          = :left,
+    RIGHT         = :right,
+    BOTTOM_LEFT   = :bottom_left,
+    BOTTOM        = :bottom,
+    BOTTOM_RIGHT  = :bottom_right
+  ].freeze
+
+  attr_accessor :row, :col
+
+  def initialize(row_or_cell_ref, col = nil)
+    if col
+      # Position.new(1, 5) のような呼び出し
+      @row = row_or_cell_ref
+      @col = col
+    else
+      # Position.new('f7')のような呼び出し
+      @row = ROW.index(row_or_cell_ref[1])
+      @col = COL.index(row_or_cell_ref[0])
+    end
+  end
+
+  def invalid?
+    row.nil? || col.nil?
+  end
+
+  def out_of_board?
+    !((0..7).cover?(row) && (0..7).cover?(col))
+  end
+
+  def stone_color(board)
+    return nil if out_of_board?
+
+    board[row][col]
+  end
+
+  def to_cell_ref
+    return '盤面外' if out_of_board?
+
+    "#{COL[col]}#{ROW[row]}"
+  end
+
+  def next_position(direction)
+    case direction
+    when TOP_LEFT     then Position.new(row - 1, col - 1)
+    when TOP          then Position.new(row - 1, col)
+    when TOP_RIGHT    then Position.new(row - 1, col + 1)
+    when LEFT         then Position.new(row,     col - 1)
+    when RIGHT        then Position.new(row,     col + 1)
+    when BOTTOM_LEFT  then Position.new(row + 1, col - 1)
+    when BOTTOM       then Position.new(row + 1, col)
+    when BOTTOM_RIGHT then Position.new(row + 1, col + 1)
+    else raise 'Unknown direction'
+    end
+  end
+end
diff --git a/reversi b/reversi
deleted file mode 160000
index 165d5aa..0000000
--- a/reversi
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 165d5aafaeaeba50c855ea1f6998e0170bca8474
diff --git a/reversi.rb b/reversi.rb
new file mode 100644
index 0000000..5667776
--- /dev/null
+++ b/reversi.rb
@@ -0,0 +1,58 @@
+# frozen_string_literal: true
+
+require_relative './reversi_methods'
+
+class Reversi
+  include ReversiMethods
+
+  QUIT_COMMANDS = %w[quit exit q].freeze
+
+  def initialize
+    @board = build_initial_board
+    @current_stone = BLACK_STONE
+  end
+
+  def run
+    loop do
+      output(@board)
+
+      if finished?(@board)
+        puts '試合終了'
+        puts "白○:#{count_stone(@board, WHITE_STONE)}"
+        puts "黒●:#{count_stone(@board, BLACK_STONE)}"
+        break
+      end
+
+      unless placeable?(@board, @current_stone)
+        puts '詰みのためターンを切り替えます'
+        toggle_stone
+        next
+      end
+
+      print "command? (#{@current_stone == WHITE_STONE ? '白○' : '黒●'}) > "
+      command = gets.chomp
+      break if QUIT_COMMANDS.include?(command)
+
+      begin
+        if put_stone(@board, command, @current_stone)
+          puts '配置成功、次のターン'
+          toggle_stone
+        else
+          puts '配置失敗、ターン据え置き'
+        end
+      rescue StandardError => e
+        puts "ERROR: #{e.message}"
+      end
+    end
+
+    puts 'finished!'
+  end
+
+  private
+
+  def toggle_stone
+    @current_stone = @current_stone == WHITE_STONE ? BLACK_STONE : WHITE_STONE
+  end
+end
+
+Reversi.new.run if __FILE__ == $PROGRAM_NAME
diff --git a/reversi_methods.rb b/reversi_methods.rb
new file mode 100644
index 0000000..6550e34
--- /dev/null
+++ b/reversi_methods.rb
@@ -0,0 +1,95 @@
+# frozen_string_literal: true
+
+require_relative './position'
+
+module ReversiMethods
+  WHITE_STONE = 'W'
+  BLACK_STONE = 'B'
+  BLANK_CELL = '-'
+
+  def build_initial_board
+    # boardは盤面を示す二次元配列
+    board = Array.new(8) { Array.new(8, BLANK_CELL) }
+    board[3][3] = WHITE_STONE # d4
+    board[4][4] = WHITE_STONE # e5
+    board[3][4] = BLACK_STONE # d5
+    board[4][3] = BLACK_STONE # e4
+    board
+  end
+
+  def output(board)
+    puts "  #{Position::COL.join(' ')}"
+    board.each_with_index do |row, i|
+      print Position::ROW[i]
+      row.each do |cell|
+        case cell
+        when WHITE_STONE then print ' ○'
+        when BLACK_STONE then print ' ●'
+        else print ' -'
+        end
+      end
+      print "\n"
+    end
+  end
+
+  def copy_board(to_board, from_board)
+    from_board.each_with_index do |cols, row|
+      cols.each_with_index do |cell, col|
+        to_board[row][col] = cell
+      end
+    end
+  end
+
+  def put_stone(board, cell_ref, stone_color, dry_run: false)
+    pos = Position.new(cell_ref)
+    raise '無効なポジションです' if pos.invalid?
+    raise 'すでに石が置かれています' unless pos.stone_color(board) == BLANK_CELL
+
+    # コピーした盤面にて石の配置を試みて、成功すれば反映する
+    copied_board = Marshal.load(Marshal.dump(board))
+    copied_board[pos.row][pos.col] = stone_color
+
+    turn_succeed = false
+    Position::DIRECTIONS.each do |direction|
+      next_pos = pos.next_position(direction)
+      turn_succeed = true if turn(copied_board, next_pos, stone_color, direction)
+    end
+
+    copy_board(board, copied_board) if !dry_run && turn_succeed
+
+    turn_succeed
+  end
+
+  def turn(board, target_pos, attack_stone_color, direction)
+    return false if target_pos.out_of_board?
+    return false if target_pos.stone_color(board) == attack_stone_color
+    return false if target_pos.stone_color(board) == BLANK_CELL
+
+    next_pos = target_pos.next_position(direction)
+    if (next_pos.stone_color(board) == attack_stone_color) || turn(board, next_pos, attack_stone_color, direction)
+      board[target_pos.row][target_pos.col] = attack_stone_color
+      true
+    else
+      false
+    end
+  end
+
+  def finished?(board)
+    !placeable?(board, WHITE_STONE) && !placeable?(board, BLACK_STONE)
+  end
+
+  def placeable?(board, attack_stone_color)
+    board.each_with_index do |cols, row|
+      cols.each_with_index do |cell, col|
+        next unless cell == BLANK_CELL
+        position = Position.new(row, col)
+        return true if put_stone(board, position.to_cell_ref, attack_stone_color, dry_run: true)
+      end
+    end
+    false
+  end
+
+  def count_stone(board, stone_color)
+    board.flatten.count { |cell| cell == stone_color }
+  end
+end
diff --git a/reversi_methods_test.rb b/reversi_methods_test.rb
new file mode 100644
index 0000000..0a17589
--- /dev/null
+++ b/reversi_methods_test.rb
@@ -0,0 +1,164 @@
+# frozen_string_literal: true
+
+require 'minitest/autorun'
+require_relative './reversi_methods'
+
+class ReversiMethodsTest < Minitest::Test
+  include ReversiMethods
+
+  def build_board(board_text)
+    board = build_initial_board
+    board_text.split("\n").each_with_index do |row, i|
+      row.each_char.with_index do |cell, j|
+        board[i][j] = cell
+      end
+    end
+    board
+  end
+
+  def test_invalid_position
+    board = build_initial_board
+    e = assert_raises RuntimeError do
+      put_stone(board, 'x0', BLACK_STONE)
+    end
+    assert_equal '無効なポジションです', e.message
+  end
+
+  def test_already_have_a_stone
+    board = build_initial_board
+    e = assert_raises RuntimeError do
+      put_stone(board, 'd5', BLACK_STONE)
+    end
+    assert_equal 'すでに石が置かれています', e.message
+  end
+
+  def test_put_stone
+    board = build_initial_board
+    assert put_stone(board, 'e6', BLACK_STONE)
+    assert_equal build_board(<<~BOARD), board
+      --------
+      --------
+      --------
+      ---WB---
+      ---BB---
+      ----B---
+      --------
+      --------
+    BOARD
+    assert put_stone(board, 'f4', WHITE_STONE)
+    assert_equal build_board(<<~BOARD), board
+      --------
+      --------
+      --------
+      ---WWW--
+      ---BB---
+      ----B---
+      --------
+      --------
+    BOARD
+  end
+
+  def test_cannot_put_stone
+    initial_data = <<~BOARD
+      W-WWWW--
+      W-BWWW--
+      WBWWWWW-
+      WWBWWW--
+      WBBBBB--
+      --B-----
+      --B-----
+      --B-----
+    BOARD
+    board = build_board(initial_data)
+    refute put_stone(board, 'b1', BLACK_STONE)
+    assert_equal build_board(initial_data), board
+  end
+
+  def test_turn
+    board = build_board(<<~BOARD)
+      --------
+      ---B----
+      --WB----
+      --WBB---
+      --WWW---
+      --------
+      --------
+      --------
+    BOARD
+    assert put_stone(board, 'b4', BLACK_STONE)
+    assert_equal build_board(<<~BOARD), board
+      --------
+      ---B----
+      --BB----
+      -BBBB---
+      --WWW---
+      --------
+      --------
+      --------
+    BOARD
+  end
+
+  def test_finished_of_initial_board
+    board = build_initial_board
+    refute finished?(board) # 初期盤面
+  end
+
+  def test_finished_of_full_board
+    assert finished?(build_board(<<~BOARD)) # 全て埋まった盤面
+      WWWWWWWW
+      WBBWWBWB
+      WBBBBWBB
+      WBWBBBBB
+      WBWWBBBB
+      WBWWWBBB
+      WWWWWWBB
+      WBBBBBBB
+    BOARD
+  end
+
+  def test_finished_of_quickest_win_board
+    assert finished?(build_board(<<~BOARD)) # 白最短勝利
+      --------
+      ---W----
+      ---WW---
+      -WWWWW--
+      ---WWW--
+      ---WWW--
+      --------
+      --------
+    BOARD
+    assert finished?(build_board(<<~BOARD)) # 黒最短勝利
+      --------
+      --------
+      ----B---
+      ---BBB--
+      --BBBBB-
+      ---BBB--
+      ----B---
+      --------
+    BOARD
+  end
+
+  def test_finished_of_player_skip_board
+    refute finished?(build_board(<<~BOARD)) # 白配置可・黒配置不可
+      WWWWWWWB
+      WBBWWBWB
+      WBBBBWBB
+      WBWBBBB-
+      WBWWBBBB
+      WBWWWBBB
+      WWWWBWBB
+      WBBBBBBB
+    BOARD
+    refute finished?(build_board(<<~BOARD)) # 白配置不可・黒配置可
+      WWWWWWWW
+      WBBWWBWB
+      WBBBBWBB
+      WBWBBBBB
+      WBWWBBBB
+      WBWWWBBB
+      BBBB-WBB
+      WBBBBBBB
+    BOARD
+  end
+end