 state (when the flag STF0 is set to "0"), the judgment result at Step SB00 is "YES". The sound source 00 proceeds to Step SB00. At Step SB00, the sound source 00 judges whether the flag STF0 is set to the impact waveform data reading state (whether the flag STF0 is set to "0"). When the flag STF0 is set to the impact waveform data reading stopped state, the judgment result is "NO". The sound source 00 proceeds to Step SB00, described hereafter. At the same time, when the flag STF0 is set to the impact waveform data reading state, the judgment result at Step SB00 is "YES". The sound source 00 proceeds to Step SB00 and starts reading the impact waveform data. Next, at Step SB00, the sound source 00 