To overcome the problem that, in the conventional venting method of an ejector pin, for performing exhaust with the outer circumferential part as a passage, owing to the sticking of vaporized decomposed products generated from a gas and an additive, and burrs generated in the case a gas vent is large, the clearance of a sliding part is clogged, thus a working defect is produced.

By locating the passage of venting inside an ejector pin not being close to a sliding part, the exhaust of a gas is performed without causing a problem of a working defect.