Cancellation of transmission of print data from a host computer to a printer is implemented under a USB Printer Class protocol without increasing the burden upon a printer on the receiving side. Specifically, when the host computer issues a transmit-abort request to the printer during the transmission of print data, the host computer notifies the printer of cancellation of data transmission via a channel different from that used for the data transmission. Upon being so notified, the printer suspends processing and discards the data that has been received. After suspending the processing, discarding the received data completing the preparation of receiving next data stream the printer notifies the host computer of the completion of abort procedure. Host computer will not send next data stream until receiving the completion of abort procedure from printer.