PURPOSE: To suppress frictional heat generated at the time of the sliding of a plunger efficiently and uniformly and prevent deterioration caused by oxidation, etc., by providing heat-radiation preventing members and anti-friction members which are in a ring shape on the sliding face of a plunger in a cylinder.

CONSTITUTION: Heat-radiation preventing members and anti-friction members which are in a ring shape are provided on the sliding face of a plunger 13 in a cylinder 12. In this case, for example, a seal is used as the heat radiation preventing member 15, and, e.g., a synthetic material composed of carbon and Teflon is used as the anti-friction member 16. The positioning of the members 15, 16 is performed by a middle piece 19 via a distance spring 18, etc. Frictional heat which is generated when the plunger 13 slides inside the cylinder 12 is suppressed by the anti-friction members 16 while being absorbed by the heat- radiation preventing members 15. Thereby, heat generation can be efficiently suppressed while being made uniform, also preventing deterioration caused by oxidation, etc.