PURPOSE: To realize the simplification of the up and down motion mechanism of a main plunger by a method wherein the main plunger is moved up and down by changing-over an oil passage by means of a change-over cylinder for working oil passage, which is elevatingly provided at the upper part of a pump casing.

CONSTITUTION: When the main plunger 20 of a pump body Ea lowers down to reach its lower end, the upper end face of the second hydraulic piston 22 of the main plunger 20 clears the oil hole 51 of a spacer 50. As a result, the pressure of working oil exerts upward on the lower end face of the change-over cylinder 30 for working oil passage, resulting in raising the change-over tube 30. Accordingly, the working oil which was pushing downward the plunger 20 pushed in an upper oil chamber 12, is discharged through a first oil passage 10a and the recess 31 of the change-over cylinder 30 from a port 14 to a discharge port. In such a manner as mentioned above, the main plunger can be moved up and down by the simple up and down motion mechanism.