While a player is selectively depressing and releasing keys of a master musical instrument, the real key movements are expressed by pieces of key motion data, and physical quantity of keys are presumed at a time later than the present time by a time period equal to communication time lag on the key trajectories determined on the basis of the pieces of key motion data; the presumed physical quantity is transmitted to a slave musical instrument through the internet, and the key movements are reproduced on the basis of the presumed physical quantity so that the performance on the master musical instrument is synchronized with that on the slave musical instrument.