A data synthesis apparatus detects the start of a period of voice waveform data, stores the voice waveform data in a first storage device, starting with its part indicative of the start of the detected period. The apparatus stores in a second storage device musical-sound waveform data including information on pulses having a specified period, and then performs a convolution operation on the voice waveform data stored in the first storage device and the musical-sound waveform data stored in the second storage device, thereby outputting synthesized waveform data synchronized with the specified period of the musical-sound waveform data stored in the second storage device.