PURPOSE: To reduce the cost by providing concaves to contain return parts on the outline of a seat at least on one of the rear end face of a plunger and the top face of a center core.

CONSTITUTION: A seat 6 is furnished in a gap 5 between a rear end face 1a of a plunger 1 and a top face 2b of a center core 2. Concave escapes of grooves 17 and 18 are provided on the side of the inner circumference of end faces 1a and 1b while gains 20 and 21 are furnished on the outer circumference. The seat 6 is furnished as it is punched by a press, that is, with returns 7. This construction permits to reduce the cost without giving any influence to the adjusting accuracy of the gap and the characteristic of a solenoid.