A synthetic resin cap is attached to the tip portion of a movable iron core of a solenoid operation portion, a sliding portion extending to the position of a magnetic plate is formed in the base end portion of the cap, a guiding concave step portion having a depth equal to or less than one half of the thickness of the magnetic plate is formed in the inner peripheral portion of the magnetic plate, and the movable iron core and the magnetic plate being not in contact with each other are held by the sliding portion fitted into the concave step portion so as to freely slide.