Abstract: A fuel injection pump for injecting compressed fuel to an engine through a reciprocative motion of a plunger includes a vertically-elongated plunger barrel, and a plunger having a shaft portion which is vertically and reciprocatively slidable along the inner space of the plunger barrel, at least one of the inner wall of the plunger barrel and the peripheral surface of the shaft portion of the plunger being formed with an oil groove for temporarily storing oil which ascends from a cam chamber through a reciprocative motion of the plunger, wherein the oil groove has at least two side walls one at the upper side of the oil groove and one at the lower side of the oil groove, and wherein lower side wall is downwardly slanted to return the stored oil to the cam chamber.