A method of generating a tone waveform using a CPU is provided which prevents or minimizes operational delays of other software processing that is executed concurrently with tone waveform generating processing on a multitask basis. The CPU collectively calculates 128 (one block of) tone waveform sample values each corresponding to a sampling clock pulse, and transmits the calculated tone waveform sample values to a reproduction section in response to a predetermined calculation triggering clock pulse generated every 128 samples. When sufficient processing capability of the CPU performing the multitask is available for the waveform sample calculation, tone waveform sample values for one or more following blocks are also calculated and stored in a sample buffer in advance. When the CPU is too busy with the other software processing to execute the waveform sample calculation, it is just sufficient that the previously stored tone waveform sample values be read out to be transmitted to the reproduction section. This prevents operational delays of the other software processing.