In response to a readout instruction, tone reproducing digital data are read out which include information indicative of tone generating events and time lengths. Then, degrading processing is performed to change the information indicative of tone generating events and time lengths, included in the read-out tone reproducing digital data, in accordance with payment of a necessary charge for the digital data, and then the changed tone reproducing digital data are output. By thus performing the degrading processing on the read-out tone reproducing digital data in accordance with the payment of the necessary charge when the digital data are to be transferred or reproduced, there can be achieved an advantageous effect of reliably preventing unauthorized use of the digital data.