PURPOSE: To switch the pressure in a vessel between pressurization and decompression by fitting a cylinder on a small-dia. part at the bottom of a bellows cylinder in a pump body in such a way as closely and rotatably, and by allowing this double cylindrical construction solely to perform the valve switching operation.

CONSTITUTION: A cylinder 10 is fitted on the small dia. part 8 at the bottom of a bellows cylinder 6 in such a way as closely and rotatably to constitute a double cylindrical construction, wherein the cylinder 10 in double cylindrically constructed part is furnished at its upper half part with a suction mouth 11 and exhaust mouth 12, both being equipped with a check valve 13a, 13b, and the small dia. part 8, on the other hand, is furnished with communication holes 14, 16a, 16b in communication with either of said suction mouth 11 of exhaust mouth and also at its lower half part with a pair of check valves 15a, 15b facing each other. Communication holes 16a, 16b are provided at least at the top of said check valve 15a, and also between the two check valves 15a, 15b, while the cylinder 10 is equipped with by-pass holes 17a, 17b as vents. Here check valves 13a, 15a or those 13b, 15b are put in operation by switching the small dia. part 8.