A magnetic armature is attracted and driven by a magnetic force which is intermittently generated between magnetic poles, and a piston reciprocated by being pushed back by a coil spring is not rotated by the coil spring. When the magnetic armature (28) as attracted between the magnetic pole members (10,12) by the magnetic force comes to a predetermined rotational angle position about the axis, the armature receives a rotational torque that is derived from the magnetic force and acts in a direction opposite to that of the rotational torque applied by a coil spring (30), thereby preventing the armature from being rotated in the predetermined direction. More specifically, the armature (28) has a circular cross-section as a whole and has a chamfered part (28′) parallel to the axis. When the chamfered part enters between the magnetic pole members, the armature receives a rotational torque that is derived from the magnetic force.