PURPOSE: To improve the extent of pump efficiency, by energizing a vertically shiftable cylinder to be pressed to the side of a discharge valve at the upper part by dint of a compression spring, while setting an outside part of this cylinder down to a suction chamber.

CONSTITUTION: A converging part 3, having a discharge hole, a discharge valve 24 and a main passage 21, is attached to the upper part of a pump casing 2. On the other hand, inside the pump casing 2, there is provided with a tubular cylinder 15 internally installing a piston 14 shiftably up and down, and this cylinder 15 is energized to be pressed to the side of a converging part 3 by dint of a compression spring 17. When this piston 14 goes upward, a fluid inside the cylinder is discharged to the main passage 21 via the discharge valve 24, but when the piston 14 goes downward, the cylinder 15 goes down against elastic force of the compression spring 17 by negative pressure inside the cylinder 15, thus there is produced a clearance between the piston and the converging part 3. With this constitution, the fluid is sucked in the cylinder 15 from a pump part 5 so that pumping operation is carried out. According to this constitution, dead capacity at a time when the piston 14 is in a top dead center is reduced, thus pump efficiency is improved.