To reduce a calculation burden while ensuring reality of sound output.

A game screen begins to be displayed and, at the same time, four kinds of sounds S1 to S4, each corresponding to its object corresponding position CP, are compositely reproduced. For example, the sound S1 starts to smoothly increase a sound level from zero level to a peak level, then returns to the zero level, and then smoothly increases again to the peak level, and returns to zero lever, corresponding to its object corresponding position CP. The sound S2 to S4, as well, each corresponding to its object corresponding position CP, are set to different volume level changes individually. The sounds S1 to S4 etc. include background music, and environmental sounds such as water sound, chirps of living creatures, wind sound, and so on.