PROBLEM TO BE SOLVED: To provide means for generating a sound waveform signal similar to a sound waveform signal of a normal idiophonic percussion instrument from a sound waveform signal of a muted idiophonic percussion instrument.SOLUTION: Each of a plurality of sound waveform signal generation units 111 includes feedback-type comb-shaped filters provided according to each of a plurality of frequency bands, and includes a damper 1113, an HSF 1114, and an LSF 1115 on a path of feedback signals. A target gain specification unit 1141 and a gain increasing/decreasing unit 1142 control gain of the damper 1113 so that a reverberation time becomes longer as an amplitude is smaller on the basis of any one of an input signal and an output signal having a larger amplitude. A filter property specification unit 1143, an HSF gain control unit 1144, and an LSF gain control unit 1145 control parameters of the HSF 1114 and LSF 1115 so that amplitudes of a mid-low range and a high range attenuate more quickly than an amplitu
