To achieve easier adjustment of the degree of difficulty in a game while reproducing real actions of a character.

A seesaw or the like varying in angle to the horizontal plane is included as a moving area of a character in three-dimensional image data. The angle of sliding at which the character begins to slide from the moving area and a friction coefficient μ are previously set according to the state of the moving area. When the character shifts onto the moving area, the angle θof an action surface at the position is calculated (S201). If the results exceed the angle of sliding (S202, YES), a dropping velocity v0 of the character is determined by the calculation of v0+μgisnθ (v0: preceding dropping velocity, g: gravity acceleration) (S203). The position of the character in sliding down with an inclination of the moving area is determined according to the velocity obtained (S204). When the results are smaller than the angle of sliding (S202; NO), the character will not drop.