PURPOSE: To boost an oil pressure again after releasing by installing a plunger member which can increase/decrease the volume of an oil chamber communicated to an actuator in a check valve provided with an operation part which can open the valve from the outside.

CONSTITUTION: A check valve 43 is opened by the manual operation of the valve opening operation part 51a of a check valve unit 20 and an oil pressure is released to the oil chamber 62 of a pump mechanism 60 from an actuator port 41a through the check valve 43. Then, a plunger member 63 is moved leftward and oil is filled in the oil chamber 62 by the elastic deformation of a compression spring 64, but as the compression spring 64 is a weak spring for the oil pressure of a prescribed very low pressure, a circuit oil pressure is dropped up to the nearly prescribed very low pressure. Then, the plunger operation part 63c of a boost operation mechanism 70 is moved pushingly rightward and the oil pressure is supplied to the actuator port 41a from the oil chamber 62 through the check valve 43 and the plunger 63 is stopped when the oil chamber 62 becomes a set low oil pressure and the oil pressure circuit holds the set low oil pressure.