A cut-off circuit diagnostic device comprising: a motor control module that is configured to control a motor; and an external safety module that is detachably attached to the motor control module, wherein the motor control module includes a motor drive unit that has an inverter circuit for controlling a voltage applied to the motor by pulse width modulation control and a drive circuit for driving a power element of the inverter circuit, a cut-off circuit that is configured to cut off power supply to the drive circuit, and a first cut-off circuit diagnostic unit that has a first diagnosis pulse generator for generating a first signal to control the cut-off by the cut-off circuit and that is configured to detect whether or not the power supply to the drive circuit is cut off according to the first signal, and the external safety module includes a second cut-off circuit diagnostic unit that has a second diagnosis pulse generator for generating a second signal to control the cut-off by the cut-off circuit and that is configured to detect whether or not the power supply to the drive circuit is cut off according to the second signal, characterized in that the motor control module further includes an external safety module connection or disconnection determination unit that is configured to determine whether or not the external safety module is attached to the motor control module, and a function switching unit that is configured to supply the second signal to the cut-off circuit when the external safety module is attached to the motor control module, and to supply the first signal to the cut-off circuit when the external safety module is not attached to the motor control module, based on the determination by the external safety module connection or disconnection determination unit, the external safety module connection or disconnection determination unit includes at least one of a connection notification receiver and a communication interface unit (13a, 13b), the connection notification receiver receiving a connection notification signal transmitted from the external safety module when the external safety module is attached to the motor control module and performing the determination based on whether the connection notification signal is received, the communication interface unit (13a, 13b) being capable of communicating with the external safety module when the external safety module is attached to the motor control module and performing the determination based on whether or not there is communication with the external safety module. a motor control module that is configured to control a motor; and an external safety module that is detachably attached to the motor control module, wherein the motor control module includes a motor drive unit that has an inverter circuit for controlling a voltage applied to the motor by pulse width modulation control and a drive circuit for driving a power element of the inverter circuit, a cut-off circuit that is configured to cut off power supply to the drive circuit, and a first cut-off circuit diagnostic unit that has a first diagnosis pulse generator for generating a first signal to control the cut-off by the cut-off circuit and that is configured to detect whether or not the power supply to the drive circuit is cut off according to the first signal, and a motor drive unit that has an inverter circuit for controlling a voltage applied to the motor by pulse width modulation control and a drive circuit for driving a power element of the inverter circuit, a cut-off circuit that is configured to cut off power supply to the drive circuit, and a first cut-off circuit diagnostic unit that has a first diagnosis pulse generator for generating a first signal to control the cut-off by the cut-off circuit and that is configured to detect whether or not the power supply to the drive circuit is cut off according to the first signal, and the external safety module includes a second cut-off circuit diagnostic unit that has a second diagnosis pulse generator for generating a second signal to control the cut-off by the cut-off circuit and that is configured to detect whether or not the power supply to the drive circuit is cut off according to the second signal, characterized in that the motor control module further includes an external safety module connection or disconnection determination unit that is configured to determine whether or not the external safety module is attached to the motor control module, and a function switching unit that is configured to supply the second signal to the cut-off circuit when the external safety module is attached to the motor control module, and to supply the first signal to the cut-off circuit when the external safety module is not attached to the motor control module, based on the determination by the external safety module connection or disconnection determination unit, the external safety module connection or disconnection determination unit includes at least one of a connection notification receiver and a communication interface unit (13a, 13b), the connection notification receiver receiving a connection notification signal transmitted from the external safety module when the external safety module is attached to the motor control module and performing the determination based on whether the connection notification signal is received, the communication interface unit (13a, 13b) being capable of communicating with the external safety module when the external safety module is attached to the motor control module and performing the determination based on whether or not there is communication with the external safety module.