PURPOSE: To secure such a pump that consists of a simple structure and the required ripple factor, by installing a first displacement chamber having a suction valve and a discharge valve as well as installing a second displacement chamber being interconnected to the discharge pipe line connected to this chamber and provided with no valve.

CONSTITUTION: When a drive shaft is rotated, pistons 2b and 3b to be slidingly contacted with a cam 1a are reciprocated, and displacement chambers 2g and 3f come to repeat both expansion and compression strokes. And at this expansion stroke of the displacement chamber 2b, a hydraulic fluid in a reservoir 14 is sucked in the displacement chamber 2g via a suction valve 2e, and the fluid inside the chamber 2g is discharged out to a discharge pipe line 5 via a discharge valve 2f at the compression stroke. And, at the displacement chamber 3f, these expansion and compression strokes are repeated in a state of phase shifting as far as 180° with the displacement chamber 2g, and while the discharge valve 2f is being closed, the fluid previously accumulated in the chamber 3f is designed so as to be sucked out to the discharge pipe line 5.