PURPOSE: To quickly detect the breakage of a working tool held by a main spindle on a machine tool.

CONSTITUTION: A breakage detecting device 100 is provided in the axially moving passage of a main spindle 56. When a detecting element 116 is moved in the direction crossing the axially moving passage in this position, a working tool is found to be not broken by the contact of the detecting element 116 with the top end of the working tool. Thus, the top end part is lost even if the working tool is broken in any point, whereby the present of a breakage can be detected according to whether the detecting element 116 makes contact with the top end part of the working tool or not.