A CPU 19a supplies parameters on musical tone signals to a tone generator 17 having a plurality of tone generation channels CH0, CH1, . . . , CH127 each generating a musical tone signal. The parameters include channel information which designates one or more of the tone generation channels, and musical tone information which defines respective musical tone signals which are to be generated in the respective tone generation channels designated by the channel information. The tone generator 17 has a tone generation reservation circuit 17b which makes the designated tone generation channels start generation of the musical tone signals defined by the musical tone information when the respective tone volume levels of musical tone signals currently generated in the tone generation channels designated by the channel information are equal to or below a certain tone volume level.