A detection portion on a sensor holder attached to a sensor mounting base is arranged to be opposed to an outer periphery of a sensor gear at a predetermined distance. The sensor holder has a holder body which holds the detection portion and a portion to be fastened which is fastened to the sensor mounting base by screws. The portion to be fastened is provided integrally with one side of the holder body that is located to face opposite the sensor gear. At least the portion to be fastened of the sensor holder is made of a material having an elastic modulus lower than the materials of the sensor mounting base and the screws.