The weight of a piston is, by shortening of a length of the piston and with the addition of a stationary core is reduced, but without reduction of a diameter of the piston. Furthermore, an inclination of the piston in a coil body is reduced, thereby to prevent the piston with a passage opening of a end yoke comes into contact. Since the weight of the piston without a reduction in the diameter of the piston can be reduced, an attenuation of an attractive force by means of a reduction of a coupled magnetic flux (diameter reduction) is brought about, as well as an impairment of the release operation efficiency due to an increase in the case of a magnetic resistance to be prevented, and fluctuations in the case of a force of attraction and a frictional force, by means of reduction in an inclination in the case of the piston is suppressed, so that a release operation characteristic can be improved.
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