The game apparatus (1) has a fixed main unit (100) and a movable operating device (8) that is held by a player and is capable of detecting the body motion of the player and of transmitting a signal to, receiving from, the main unit (100). The operating device (8) has an acceleration sensor (86) for detecting acceleration, a light source (85), and a transmitter (87) for transmitting a value detected by the acceleration sensor (86) to the main unit (100). The main unit (100) has light receiving units (9L,9R) that receive the light emitted from the light source (85) to detect the intensity of the received light, a RAM (14), and a processor (16). Processor (16) repeats a process of detecting the position of the operating device (8) based on the intensity of light detected by each of the light receiving units (9L,9R), causes a RAM (14) to store the detected position history, determines whether a player has performed a predetermined motion based on the position history and the detected value of the acceleration sensor (86).