PURPOSE: To obtain a heavier cutting by utilizing a wedge in order to clamp a tool securely to the main spindle of a machine tool.

CONSTITUTION: To clamp the tool 2 inserted into the main spindle 1, a three position cylinder 8 is moved forward so that the wedge 7 spreads out crank levers 6 to draw a drawbar 4, and pawls 3 hold a pull stud 2a while pulling. When the three position cylinder 8 moves forward through its length of travel, the forces amplified by the tapered surfaces of the wedge 7 and by the leverage of the crank levers 6 permit the produce of a clamp force much stronger than the insertion force of the wedge 7 into the crank levers 6. Moreover, the wedge effect substantially elimintaes looseness or the like due to the external force applied to the tool 2.