In a case where a select communication with an apparatus main body is performed, an option side controller selected by a select signal determines whether a command-save-bit is true or not. In a case where the command-save-bit is not true, the option side controller executes a processing in accordance with the command signal. In a case where the command-save-bit is true, the option side controller stores contents of the command signal into a memory. In a case where the simultaneous communication with the apparatus main body is performed, all of the option side controllers read the command signal from the memory and executes a processing in accordance with the command signal.