PURPOSE: To increase a peak value of a discharge pressure in a plunger pump which intermittently injects water with high pressure while utilizing pulsative motion of discharge pressure together with a suction/discharge cycle.

CONSTITUTION: A pump chamber 28, varied inits volume by reciprocation of a plunger 26, is communicated with a storage chamber 60. A piston 62 is energized in a direction of decreasing the volume of the storage chamber 60 by a compressive coil spring 68. A secondary discharge valve 70 always interrupts the connection between the storage chamber 60 and a secondary discharge port 56 by means of a compressive coil spring 72. An operation bar 76 has a base end screwed to a leading end of the plunger 26, and has a leading end in abutment against a secondary discharge valve 70 in the vicinity of a top dead center of the plunger 26, for forcibly opening the secondary discharge valve 70.