A piston pump for pressurizing a material, and feeding the pressurized material, comprising: a material compressing cylinder with a cylinder chamber having an open rear end; a plunger penetrated from the rear end into the cylinder chamber fluid-tightly, which reciprocates between a top dead center and a bottom dead center for feeding the material; a holder for supporting and fixing the material compressing cylinder with virtually horizontal inclination at a slight inclination angle (α) with respect to a center axis along a reciprocating direction of the plunger so that a head of the cylinder is disposed at a lower side in a gravity direction; a hollow cylindrical storage chamber member which is coaxially connected to the rear end of the material compressing cylinder consecutively with a cylindrical space having a larger diameter than that of the cylinder chamber, including a storage chamber for receiving the plunger, and an exhaust port for discharging the fluid in the storage chamber; a linear drive unit having a reciprocating member which linearly reciprocates at a stroke longer than a distance between the top dead center and the bottom dead center of the plunger parallel thereto, and allowing the plunger to reciprocate via the reciprocating member; an intake port disposed on the head of the material compressing cylinder upward in the gravity direction, which is switchably communicated with a material supply source, a cleaning liquid supply source, and a steam supply source; an intake flow passage for communicating the intake port with the cylinder chamber; an outlet port disposed on the head of the material compressing cylinder downward in the gravity direction; an outlet flow passage for communicating the outlet port with the cylinder chamber; a first check valve disposed in the intake flow passage; and a second check valve disposed in the outlet flow passage, wherein the storage chamber member is configured that the plunger is drawn into the storage chamber, and the cylinder chamber and the storage chamber are communicated when the reciprocating member of the linear drive unit moves to a terminal end of the stroke, the inclination angle (α) is equal to or larger than 0.5° and smaller than 5°, and the exhaust port is disposed at the lower side of the storage chamber in the gravity direction at its end closer to the material compressing cylinder. a material compressing cylinder with a cylinder chamber having an open rear end; a plunger penetrated from the rear end into the cylinder chamber fluid-tightly, which reciprocates between a top dead center and a bottom dead center for feeding the material; a holder for supporting and fixing the material compressing cylinder with virtually horizontal inclination at a slight inclination angle (α) with respect to a center axis along a reciprocating direction of the plunger so that a head of the cylinder is disposed at a lower side in a gravity direction; a hollow cylindrical storage chamber member which is coaxially connected to the rear end of the material compressing cylinder consecutively with a cylindrical space having a larger diameter than that of the cylinder chamber, including a storage chamber for receiving the plunger, and an exhaust port for discharging the fluid in the storage chamber; a linear drive unit having a reciprocating member which linearly reciprocates at a stroke longer than a distance between the top dead center and the bottom dead center of the plunger parallel thereto, and allowing the plunger to reciprocate via the reciprocating member; an intake port disposed on the head of the material compressing cylinder upward in the gravity direction, which is switchably communicated with a material supply source, a cleaning liquid supply source, and a steam supply source; an intake flow passage for communicating the intake port with the cylinder chamber; an outlet port disposed on the head of the material compressing cylinder downward in the gravity direction; an outlet flow passage for communicating the outlet port with the cylinder chamber; a first check valve disposed in the intake flow passage; and a second check valve disposed in the outlet flow passage, wherein the storage chamber member is configured that the plunger is drawn into the storage chamber, and the cylinder chamber and the storage chamber are communicated when the reciprocating member of the linear drive unit moves to a terminal end of the stroke, the inclination angle (α) is equal to or larger than 0.5° and smaller than 5°, and the exhaust port is disposed at the lower side of the storage chamber in the gravity direction at its end closer to the material compressing cylinder.