PURPOSE: To control the generation of cavitation in an expansion process by providing an internal pressure regulating mechanism for preventing a negative pressure in an inner chamber of a plunger from being lowered under the vapor pressure of a fluid in the plunger reciprocating in accordance with the rotation of a cam.

CONSTITUTION: A plunger 1 is radially inserted into a housing 2 and pressed and energized on a cam collar 6 by a spring 4 for a plunger with one end engaged with a plunger cap 3 through a retainer 5. A cam is provided inside the cam collar 6. The plunger 1 is reciprocated by the rotation of the cam 7. In this case, the retainer 5 is provided with a plunger internal pressure regulating mechanism therein. This regulating mechanism is provided with a piston oil chamber 5a formed along the center of the retainer 5 and a piston 8 fitted therein through a spring 9. When the internal pressure of the plunger is lowered, the piston 8 falls against the spring 9, so that the internal pressure of the plunger is prevented from being lowered under the vapor pressure of oil.