To maintain a feeling of tension of a game player during assist display mode.

An AT lottery part 50 determines a frequency k of transition to an AT mode. A control part 30a inputs the frequency k of transition determined by an AT lottery part 50 into a transition frequency division setting part 54. The transition frequency division setting part 54 calculates so that k=m (transition frequency to AT mode)×n (a number of sets) is satisfied, and inputs the values of m and n into the control part 30a. The AT transition lottery part 53 performs the AT transition lottery for determining whether or not the game shifts to the AT mode. Each time the transition frequency to AT mode reaches to the value m, the AT transition lottery part 53 draws lots using the AT low transition probability table 56.