When a gravity center position enters a right input area, a motion of a character in a virtual game space pushing down the right pedal of a bicycle is started, and the bicycle is accelerated. Further, a reference position is moved so as to gradually approach the gravity center position, and the boundary between a non-input area and a left input area is also moved so as to gradually approach the gravity center position in conjunction with the reference position. When the gravity center position enters the left input area, a motion of the character in the virtual game space pushing down the left pedal of the bicycle is started, and the bicycle is accelerated. Further, the reference position is moved so as to gradually approach the gravity center position, and the boundary between the non-input area and the right input area is also moved so as to gradually approach the gravity center position in conjunction with the reference position.