Abstract: A relay module, by which a time required when a motor is optionally mounted on a machine tool or removed therefrom can be omitted. The relay module comprises only a case and terminal screws of an electromagnetic contactor for a motor from which internal components are removed. The relay module is disposed when the motor is not connected and an electromagnetic switch is disposed when the motor is connected. Further, the relay module or electromagnetic switch is wired at all times. When the motor is optionally mounted on the machine tool, it suffices only to replace the relay module thereof with the electromagnetic switch and no wiring job is additionally required. Further, when the motor is removed, it suffices only to replace the electromagnetic switch with the relay module and a job for removing the wiring is not necessary.