An interface system for a copying system, wherein a frame of transmission data employing a serial communication between a copying machine main body and peripheral devices thereof comprises an address block, a data block and a checksum data block, an optional device corresponding to an address transmitted from the main body transfers data to the main body when the data received coincides with checksum data obtained in the device based on the received data and the transfer of the data is complete when the main body confirms that the transmission address coincides with the transferred data, and when the transferred data is not returned from the optional device or the coincidence is not confirmed, it is determined that a transmission error occurs and data is transmitted from the main body again, whereby reliability of transmission is improved.