The present invention generates data series indicating respective combined-wave data pieces by a first step of obtaining a reference time length as a reference of a time length of one combined wave, a sampling interval time, and a frequency fluctuation rate, a second step of calculating a total number of samples in the data series indicating the one combined wave on the basis of the reference time length, the sampling interval time, and the frequency fluctuation rate, a third step of calculating a rotation angle with respect to the sampling interval time on the basis of the total number of samples for each of plural sound data pieces, a fourth step of calculating combined values for the total number of samples, the combined values being obtained by combining respective values of the plural sound data pieces, the values being calculated on the basis of the rotation angles for the respective sampling interval times, a fifth step of generating a series of the combined values for the total number of samples for the respective sampling time intervals as a data series of the one combined-wave data piece, and performing a sequence of the processes of the second to the fifth steps by a predetermined times while changing the frequency fluctuation rate every time when the sequence of the processes is executed once.