PURPOSE: To simplify the control of the time when the data transfer of a receiving side unit is suspended, by holding a common bus for a transfer scheduled period, even in case a transmission is suspended by some reason, and preventing the start of the next new data transfer.

CONSTITUTION: A transmitting side unit U1 sends out a command CMD1 containing the number of data to be transferred to the other unit U3, unless the request of a higher priority order than itself exists, and raises a CBSY signal in order to secure a common by CBUS by the necessary time required for the transfer the number of data. The unit U3 designated as the opposite party by the command raises an RCV signal for showing a fact that a reception is being executed, by the time required for receiving the designated number of data by the command. Accordingly, even in case the transmission is suspended by some reason, the bus is turned 'on' by the same length as the time when the RCV signal is transferred normally, the right of using of the common bus CBUS is reserved, and it does not occur that the common bus CBUS is used newly. To each control signal, the priority order is given in order of signal lines CBSY, RCV, REQ1, REQ2 and REQ3.