Whether a safety function unit having a safety function of a desired standard is correctly mounted in a safety device having a configuration in which a basic
control function and safety function are separated is accurately determined. The safety device includes a controller and the safety function unit. The controller is provided with a unit which selects a category for identifying a safety function unit which should be connected, a unit which transmits a reference
signal to the safety function unit, a unit which determined, based on a category identification
signal returned from the safety function unit, whether or not the category of the connected safety function unit coincides with a selected category, and a unit which, in the event that the result of the determination is such that the category of the connected safety function unit and the selected category do not coincide, prohibits the output of a
control signal. The safety function unit is provided with a unit which, based on the reference
signal transmitted from the controller, generates the category identification signal in accordance with a process appropriate to the category of the safety function unit, and returns it to the controller.