To provide a pump capable of continuously supplying a large driving force and of being miniaturized and reduced in weight.

Air compressed by a compressor and regulated in pressure by a filter regulator is sent to a first operating chamber 26 within a cylinder 12 by a selector valve 17. When a piston 25 is pushed to the second pump tube 14 side, water within a second retention chamber 30 passes a non-return valve 42 to be discharged into a manifold 19. At the same time, water is sucked from a tank 18 to a first retention chamber 29. By switching the selector valve 17, the air is sent to a second operating chamber 27. When the piston 25 is pushed to a first pump tube 13 side, the water retaining inside the first retention chamber 29 passes a non-return valve 40, a pressure fluid pipe 39, and a nonreturn valve 41 to be discharged into the manifold 19. This operation is repeated by switching the selector valve 17.