Output data varying depending on a motion of an input device itself is obtained and stored into a memory. From the stored output data history, successive extremums of values indicated by the output data are detected. An interval between times when the output data indicating the successive extremums and an elapsed time from the detection of the extremum is measured. Thereafter, game input data is calculated by substituting into a predetermined function the measured elapsed time and the calculated time interval, and the game input data is used to execute a game process.