To provide a vacuum pump capable of executing the work to install an air-conditioner simply and quickly while ill influence upon the environment is suppressed.

The vacuum pump includes a cylinder 13 whose inside is partitioned by a piston 14 into two chambers 13A and 13B, and at the upper dead point and the lower dead point in the two chambers 13A and 13B, suction ports 18 and exhaust ports 17 are formed where exhaust check valves 17a and 17b and the suction check valves 18a and 18b are installed, respectively, wherein the suction ports 18 are coupled together by a coupling port part 11, and in whichever direction the piston is moved, the differential pressure between the two chambers decays gradually from the initial period to put the suction port side in a negative pressure condition, and a sensor is mounted to count the reversals of the piston in reciprocation. The count serves for easily presuming the attained degree of vacuum.