PURPOSE: To suppress the ill effect due to cavitation and the overload to the driving side of a pump by proper controlling the pressurized state in a chamber into which working oil is supplied in accordance with the discharge amount of paint.

CONSTITUTION: When a pump piston 18 is transferred from the top dead point state to a decompression cycle in no-discharge time, the working oil 6 in a working oil storage chamber 17 flows into the second chamber 5 through an oil suction port 20 and a supplementary passage 19, since a suction valve 23 is opened in all strokes in the decompression cycle, and the working oil 6 in nearly the same amount to the amount leaked from the second chamber 5 in compression cycle is supplied, and generation of cavitation is obviated. Therefore, even if the pump piston 18 is transferred from the bottom dead point state to the compression cycle, generation of impact and corrsion etc. due to cavitation is prevented, and further, the overload for the driving source of the pump can be suppressed, since the suction valve 23 is in the opened state except a little period in the stroke in the compression stroke.