To provide a vacuum pump capable of easily working in a short time when installing an air conditioner while restricting the bad influence to the environment.

The inside of a cylinder is divided into two chambers by an operation type piston, and a cylinder top dead point surface and a cylinder bottom dead point surface of the partitioned two chambers have an intake port and an air discharge port respectively having two check valves. The intake port side of this vacuum pump becomes a negative pressure by operating the piston while gradually reducing the pressure difference between both the two chambers from the initial time.