A plunger pump includes a pump body, a pump head attached to a tip end portion of the pump body, the pump head including a pump chamber, an inlet path and an outlet path, a plunger that slides in an axial direction along the inner wall of the pump chamber, and a ring-shaped plunger seal for preventing fluid leakage from the pump chamber to the pump body. The plunger seal is arranged between the pump chamber and the pump body with a front surface facing the pump chamber and a back surface facing the pump body, includes a penetration hole from the front surface to the back surface, and holds the plunger in a slidable manner with the inner wall of the penetration hole. The plunger seal includes a ring-shaped hollow portion surrounding the penetration hole, the hollow portion communicating with a back surface side of the plunger seal and not communicating with a front surface side of the plunger seal. An elastic member for expanding the hollow portion in a radial direction of the plunger by an elastic force is provided inside the hollow portion.