A cam full close stopper defines a cam full close position that is a limit position of a rotatable range of a cam. A sensor element outputs a signal corresponding to a rotation angle of the cam. A signal processor changes the signal output from the sensor element into a sensor output. A storage part memorizes a data table representing a correspondence relationship between the rotation angle of the cam and the sensor output of the signal processor in a predetermined form, characteristics of the sensor output being adjustable at a plurality of points with respect to the rotation angle of the cam. The storage part memorizes the sensor output of the signal processor when the cam is fully closed at the cam full close position.