PURPOSE: To increase the data transfer speed without reqiring any operation to the transmitted data in a data transfer system where the data are transferred between a control part consisting of a microcomputer and a peripheral device working under the control of the control part via a serial port.

CONSTITUTION: A control part keeps the non-active state of a transfer control signal CNT for a period longer than the access cancel informing time T1 to inform all peripheral devices of the access cancel state. Then, the signal CNT is kept active for a prescribed access request informing time T2 so that an access request is reported. The signal CNT is set in a non-active state again with the access request. This non-active state is kept for a period shorter than the time T1 and equal to a period of duration that is set in response to the address of the peripheral device to which the data are transferred. Thus, this peripheral device, i.e., the data transfer destination is esignated. Then, the data are transferred to the designated peripheral device via a serial data line.