PURPOSE: To improve efficiency of a pump through a simple constitution, by a method wherein a fixed piston at the lower end of a first rod engages with a movable cylinder at the lower end of a second rod, and a suction valve installed to the cylinder is opened and closed by inertia according to movement of the cylinder.

CONSTITUTION: When, with a crank 26 rotated, a slider 22 moves downward within a guide frame 50 through the medium of a connecting lever 24, a second rod 16 and a cylinder 20 move downward. A suction valve 32 is left approximately at its original position by inertia, and fluid in a storage tank 54 is sucked in a pump chamber 28 through suction ports 30. When, with the crank 26 rotated, the slider 22 moves upward through the medium of the connecting lever 24, the second rod 16 and the cylinder 20 move upward, the suction valve 32 is closed, and fluid in the pump chamber 28 pushes upward a discharge valve 36 through a passage 14 to discharge high-pressure fluid through a discharge pipe 38.