According to one embodiment, there is provide a piston pump, including: a cylinder having a bottomed cylindrical shape; a piston slidable within the cylinder, the cylinder and the piston defining a pressure chamber therebetween; a suction valve unit disposed in a suction passage of the pressure chamber; and a discharge valve unit disposed in a discharge passage of the pressure chamber, wherein the piston includes a cylindrical piston portion and a large diameter piston portion provided to cover one end of the cylindrical piston portion, and wherein the large diameter piston portion includes a working fluid guide groove extending thereinside along the axial direction of the piston.