A clamping device for clamping a cutting insert to an insert seat of a tool body, comprising: a guide hole of the tool body; a screw hole formed in a direction intersecting with the guide hole from one side surface of the tool body and connected to the guide hole; a clamp member having a clamp portion and being arranged movably in a predetermined guiding direction in the guide hole; and a screw member screwed with the screw hole and movable in first and second directions by rotation, wherein the screw member is formed capable of engagement with the clamp member in both movements in the first and second directions; the guide hole has a first sliding surface engaged with the clamp member and converting a force in the first direction received by the clamp member from the screw member into a force for moving the clamp portion on the guide hole side and in a direction toward the cutting insert, and a second sliding surface engaged with the clamp member and converting a force in the second direction received by the clamp member from the screw member into a force for moving the clamp portion on the side opposite to the guide hole and in a direction away from the cutting insert; characterized in that a seat surface of the insert seat is formed around the guide hole; the screw hole is arranged so that its central axis line is substantially parallel to the seat surface of the insert seat; the clamp member has a first engagement surface engaged when the screw member moves in the first direction and a second engagement surface engaged when the screw member moves in the second direction; the first and second engagement surfaces (4, 5) are arranged in a direction substantially orthogonal to the central axis line; the clamp member has a first contact surface engaged with the first sliding surface and a second contact surface engaged with the second sliding surface; and the first and second contact surfaces (1a, 1b) are constituted by planes and inclined with respect to the first and second engagement surfaces (4, 5), respectively. a guide hole of the tool body; a screw hole formed in a direction intersecting with the guide hole from one side surface of the tool body and connected to the guide hole; a clamp member having a clamp portion and being arranged movably in a predetermined guiding direction in the guide hole; and a screw member screwed with the screw hole and movable in first and second directions by rotation, wherein the screw member is formed capable of engagement with the clamp member in both movements in the first and second directions; the guide hole has a first sliding surface engaged with the clamp member and converting a force in the first direction received by the clamp member from the screw member into a force for moving the clamp portion on the guide hole side and in a direction toward the cutting insert, and a second sliding surface engaged with the clamp member and converting a force in the second direction received by the clamp member from the screw member into a force for moving the clamp portion on the side opposite to the guide hole and in a direction away from the cutting insert; characterized in that a seat surface of the insert seat is formed around the guide hole; the screw hole is arranged so that its central axis line is substantially parallel to the seat surface of the insert seat; the clamp member has a first engagement surface engaged when the screw member moves in the first direction and a second engagement surface engaged when the screw member moves in the second direction; the first and second engagement surfaces (4, 5) are arranged in a direction substantially orthogonal to the central axis line; the clamp member has a first contact surface engaged with the first sliding surface and a second contact surface engaged with the second sliding surface; and the first and second contact surfaces (1a, 1b) are constituted by planes and inclined with respect to the first and second engagement surfaces (4, 5), respectively.