PURPOSE: To reduce the cost by using one serial port of a copying machine main body to alternately communicate serial data among the copying machine main body, an automatic document feeder, and sorters in time division.

CONSTITUTION: A copying machine main body 2, an automatic document feeder 3, and sorters 4 to 6 are coupled by serial interfaces 9 to 13, and a signal is transmitted from the copying machine main body 2 to the automatic document feeder 3. The automatic document feeder 3 transmits information of the automatic document feeder 3 to the copying machine main body 2 after terminating reception of the signal from the copying machine main body 2. That is, the automatic document feeder 3 returns information in response to the signal from the copying machine main body 2, and next, a signal is transmitted from the copying machine main body 2 to sorters 4 to 6, and sorters 4 to 6 transmit information of sorters 4 to 6 to the copying machine main body 2 after terminating reception of the signal from the copying machine main body 2. The copying machine main body 2 communicates serial data with the automatic document feeder 3 and sorters 4 to 6 alternately in time division in this manner. Thus, the serial interface constitution advantageous with respect to cost is obtained.