A plunger main body is fixed to a shaft to reciprocate together with the shaft in an axial direction within a predetermined range. An axial overlapped surface area between a rear stator main body and the plunger main body is reduced when the plunger main body is moved from a rear stator main body side toward a front stator main body side. A plunger projection radially outwardly projects from an outer peripheral wall of an end portion of the plunger main body, which is axially located on the rear stator main body side.