PURPOSE: To improve the utilizing efficiency of a bus by separating with time the connecting operation and data transfer of optional two devices in transferring information between the optional two devices.

CONSTITUTION: When a device assigned time section 2m of a device (m) is over and a device assigned time section 2n of a device (n) reaches, the device (n) transmits a number 7a of a device (s) and a reservation signal 9. The device (s) decodes the device number 7a and the reservation signal 9 and transmits a reservation approval signal 10 to the device (n). When the reservation of data transfer between the devices (n) and (s) is finished, a bus is opened once and a data transfer request signal 5 is transmitted to the device (s) after the time 2n assigned to the device (n) is awaited. The device (s) receiving the request signal 5 transmits a data transfer approval signal 6 to the device (n) when the preparation is finished, and the device (n) transfers data 7c∼7e to its end when the approval signal 6 is obtained within the own assigned time.