A piston pump comprising: a sleeve housing which forms a bore and is provided with a discharge hole; a piston which is inserted into the bore to be reciprocally movable; an inlet valve; a cover which is connected to the sleeve housing to surround a portion where the discharge hole is formed; and an outlet valve which is configured to open/close the discharge hole, wherein the outlet valve comprises: a ball which is movably disposed between the sleeve housing and the cover to open/close selectively the discharge hole; and a ball biasing member which exerts a force to urge the ball to move to a position where the discharge hole is closed, wherein the ball biasing member comprises a body which elastically supports the ball, wherein the ball biasing member is formed such that an oil discharge passage for discharging an oil discharged via the discharge hole to an outside is formed between the sleeve housing and the cover, characterized in that the piston has an inlet passage through which oil is introduced into the bore; the inlet valve is configured to open/close the inlet passage of the piston; and wherein the ball biasing member further comprises a plurality of legs which are respectively connected to the body and extend from the body toward the sleeve housing, wherein the legs are inserted into a gap between the sleeve housing and the cover. a sleeve housing which forms a bore and is provided with a discharge hole; a piston which is inserted into the bore to be reciprocally movable; an inlet valve; a cover which is connected to the sleeve housing to surround a portion where the discharge hole is formed; and an outlet valve which is configured to open/close the discharge hole, wherein the outlet valve comprises: a ball which is movably disposed between the sleeve housing and the cover to open/close selectively the discharge hole; and a ball biasing member which exerts a force to urge the ball to move to a position where the discharge hole is closed, wherein the ball biasing member comprises a body which elastically supports the ball, wherein the ball biasing member is formed such that an oil discharge passage for discharging an oil discharged via the discharge hole to an outside is formed between the sleeve housing and the cover, characterized in that the piston has an inlet passage through which oil is introduced into the bore; the inlet valve is configured to open/close the inlet passage of the piston; and wherein the ball biasing member further comprises a plurality of legs which are respectively connected to the body and extend from the body toward the sleeve housing, wherein the legs are inserted into a gap between the sleeve housing and the cover. a ball which is movably disposed between the sleeve housing and the cover to open/close selectively the discharge hole; and a ball biasing member which exerts a force to urge the ball to move to a position where the discharge hole is closed, wherein the ball biasing member comprises a body which elastically supports the ball, wherein the ball biasing member is formed such that an oil discharge passage for discharging an oil discharged via the discharge hole to an outside is formed between the sleeve housing and the cover, characterized in that the piston has an inlet passage through which oil is introduced into the bore; the inlet valve is configured to open/close the inlet passage of the piston; and wherein the ball biasing member further comprises a plurality of legs which are respectively connected to the body and extend from the body toward the sleeve housing, wherein the legs are inserted into a gap between the sleeve housing and the cover.