To easily remove bubbles, and furthermore make the replaceability of fluid excellent by providing check valves for the side of a pump head and a plunger which have a cylinder as a column shaped space in their inside parts, and for the side of the pump head respectively, and thereby reciprocally moving the plunger within the cylinder.

In a stroke (suction stroke) that a plunger 16 is drawn out of a cylinder 12, a check valve 20 at the plunger 16 side is opened, and a check valve 14 at the side of a pump head 11 is closed. And fluid passes through a suction path 17 penetrating through a cross head 19 and the plunger 16 so as to be sucked in the inside of the cylinder 12. In a stroke (discharge stroke) that the plunger 16 is inserted into the cylinder 12, the check valve 20 at the side of the plunger 16 is closed, the check valve 14 at the side of the pump head 11 is opened, and the fluid of the cylinder 12 is discharged outside by way of a discharge path 13 and the check valve 14.