When a game mode is shifted from a base game to a free game, if a symbol of “APPLE” is rearranged in a symbol display frame 111 for the first time in one unit game which composes the free game, the payout amount which is awarded to a player is computed based on the number of the symbols of “APPLE” that are rearranged for the first time. The symbols of “APPLE” that are rearranged for the first time are displayed in the stock area 201. In a following unit game, if the symbol of “APPLE” is rearranged in a symbol display frame 111 again, the payout amount which is awarded to a player is computed based on a total number of the symbols of “APPLE” which is obtained by adding the number of the symbols of “APPLE” being displayed in the stock area 201 to the number of the rearranged symbols of “APPLE”. At this time, the rearranged symbols of “APPLE” are additionally displayed in the stock area 201 in addition to the symbols of “APPLE” being displayed in the stock area 201. The same operation will be carried out in the following unit games.