According to the present invention, in response to the change of the menu hierarchy, it is possible to diversify the BGM easily. First, BGM 1, BGM 2, and BGM 3 (hereinafter the BGM 1, 2, and 3) that are the same piece of music performed with different sounds are played back simultaneously. Only the volume of the BGM 1 is turned up and the volume of the BGM 2 and 3 are turned down to mute. Next, in response to the change of the menu hierarchy, the volume of the BGM 1 is muted and the volume of one of the muted BGM 2 or 3 is turned up.