PURPOSE: To prevent mixing of fuel with lubricating oil without using any special, lubricating oil pressure control device, and restrict the generation of wear due to seizure between a plunger and the inside peripheral surface of a cylinder.

CONSTITUTION: A seal member 88 is arranged between an oil groove 63 and a pump chamber 32, and on the basis of the sealing member 88 as a border, the length L2 of a plunger supporting part on the oil groove 63 side is set longer than the length L1 of a plunger supporting part on the pump room 32 side. Then, the clearance S2 between the plunger 33 on the oil groove 63 side and a plunger sliding hole is set smaller than the clearance S1 between the plunger 33 on the pump room 32 side and the plunger sliding hole.