To completely perform pressurizing operation at the time of low pressure by holding an elastic member arranged an exhaust valve so as to pressingly attach by force of the degree to which exhaust from an exhaust hole is not prevented in a normal operating condition, in the vicinity of the exhaust hole in a case.

In a pressurizing chamber 1, a bottom surface is formed in a nearly circular cone shape, and a conical cross sectional shape is applied thereon, in order to improve a pressurizing effect. An exhaust spring 4 is formed by a weak coil spring, the inner diameter of a part of the exhaust hole 39 is enlarged, and is held in an upper case 32. Downward pre-pressure (pressingly attaching force) of the exhaust valve 26 is applied by its compressive force. An optimal range exists in the magnitude of its spring force. Namely, at the time of just after operating start when pressurizing is not operated yet, sufficient adhesion is applied on the exhaust valve 26, pressurizing operation is started surely. On the other hand, when a pressurizing operation is advanced to a prescribed value since its operation is excessively, the exhaust valve 26 is pushed and opened, and it is selected so as to carry out expected exhaust operation.