A non-removable fixed protective cover, of protective covers of a machine tool, is fitted with a light-projecting unit and a light-receiving unit, while a removable fixed protective cover and a movable protective cover are each fitted with a reflector. The light-projecting unit, light-receiving unit, and reflector are disposed so that light emitted from the light-projecting unit reaches the light-receiving unit only when the removable fixed protective cover is mounted in a proper position and the movable protective cover is fully closed.