A spindle device for a machine tool including a hollow spindle which is rotatably supported in a housing, the spindle device comprising: an annular boss member which is affixed to the housing, which is formed so as to penetrate into the spindle, and which includes an opening formed in the axial direction of the spindle, a cooling passage formed in an outer circumferential surface of the boss member so as to face an inner circumferential surface of the spindle; a draw bar for drawing in and clamping a tool holder to the tip of the spindle, a first bearing which rotatably supports the spindle and which is arranged between a rear end of the spindle and the housing, and an unclamping device which is supported in the housing and which is provided in an annular gap between an inner circumference of the spindle and an outer circumference of the draw bar, wherein the cooling passage extends helically about an axis of the spindle, wherein the cooling passage is at least partially arranged in the same plane as the first bearing, which is perpendicular to the axis of the spindle, wherein the unclamping device is provided so as to be reciprocal along the axis of the spindle between a clamped position and an unclamped position, wherein the unclamping device includes a piston which is rotatably supported by a second bearing on the outer circumferential surface of the spindle, wherein the piston is formed so as to push the draw bar toward the front of the spindle by moving the spindle forward along the axis of the spindle from the clamped position to the unclamped position by means of fluid pressure, and wherein, when the piston is in the clamped position, the cooling passage is at least partially arranged in the same plane as the second bearing, which is perpendicular to the axis of the spindle. an annular boss member which is affixed to the housing, which is formed so as to penetrate into the spindle, and which includes an opening formed in the axial direction of the spindle, a cooling passage formed in an outer circumferential surface of the boss member so as to face an inner circumferential surface of the spindle; a draw bar for drawing in and clamping a tool holder to the tip of the spindle, a first bearing which rotatably supports the spindle and which is arranged between a rear end of the spindle and the housing, and an unclamping device which is supported in the housing and which is provided in an annular gap between an inner circumference of the spindle and an outer circumference of the draw bar, wherein the cooling passage extends helically about an axis of the spindle, wherein the cooling passage is at least partially arranged in the same plane as the first bearing, which is perpendicular to the axis of the spindle, wherein the unclamping device is provided so as to be reciprocal along the axis of the spindle between a clamped position and an unclamped position, wherein the unclamping device includes a piston which is rotatably supported by a second bearing on the outer circumferential surface of the spindle, wherein the piston is formed so as to push the draw bar toward the front of the spindle by moving the spindle forward along the axis of the spindle from the clamped position to the unclamped position by means of fluid pressure, and wherein, when the piston is in the clamped position, the cooling passage is at least partially arranged in the same plane as the second bearing, which is perpendicular to the axis of the spindle.