PURPOSE: To easily and precisely detect whether a tool has abnormality or not without causing increase in the number of parts and complexity of structure.

CONSTITUTION: It is devised to make an operating lever 9b of a limit switch 9 for abnormality detection contact with the peripheral surface of a drill D with pressure by arranging the limit switch 9 for abnormality detection on a switch holder 3 held on an actuating rod 2a of a cylinder actuator 2 for pressing in the state where the operating lever 9b faces against the peripheral surface of the drill D and by actuating the cylinder actuator 2 for pressing most extensively at a point where a drill unit Y is returned to normal.