PURPOSE: To discharge bubbles accumulated in an oil storage chamber to the outside without allowing them to flow into an inner cylinder.

CONSTITUTION: Bubbles 20 accumulated in an oil storage chamber 12 flow out from an oil supplying hole 14 before flowing into the inner cylinder 13, by making a part of the upper end opening part of an inner cylinder 13 lower than the horizontal line L passing the lower end of the oil supplying hole 14 on the uppermost part of a plunger 5 and is quickly discharged to the outside through the clearance between the plunger 5 and a body 1.