PURPOSE: To simplify the external mechanism of a high pressure pump device and make the pump compact by providing a piston stroke detecting means and a liquid delivery passage within pump body.

CONSTITUTION: An expanded chamber 2 is formed in a booster pump body 1 to constitute a hydraulic cylinder for actuating a reciprocating piston 4. Both ends of the reciprocating piston 4 are inserted in a pump cylinder 3 respectively for forming two pump chambers. A liquid suction passage 14 and a liquid delivery passage 15 having non-return valves 12 and 13 continuous to said pump chambers are provided in the pump body 1. And both ends of the expanded chamber 2 are fitted with proximity switches 16 and 16 for detecting the stroke of the reciprocating piston 4 and an electromagnetic changeover valve 8 for controlling hydraulic pressure to a hydraulic cylinder is actuated by the proximity switches 16 and 16.