PURPOSE: To quickly and accurately detect tool breakage regardless of whether a ganghead is operated or suspended by judging whether a tool is broken or not with the tool rotated or suspended by means of the tool breakage signal output at the time point when the ganghead is fitted to a tool breakage detector.

CONSTITUTION: A detection plate 1 is fixed to a bed 14 as a tool breakage detecting means opposing to a ganghead, and tool breakage detecting sections 2 is provided in this detection plate 1. The number of the tool breakage detecting section corresponds to the number of the tools 4 positioned at the head of the ganghead. The tools 4, a tool rest 5 and a motor 6 are advanced to and retracted from the detection plate 1 by the drive of the servo motor 7 connected to the table 9, as a result, the tools 4 approach and separate from the tool breakage detecting section 2. Whether tools 4 are broken or not is discriminated by an optical sensor 3 at a relative position of a tool 4 in relation to the tool breakage detecting section 2, and the discriminated signal is induced through an optical fiber 10, and the broken position of each tool 4 is discriminated by the logical product of the discriminated signal and the position signal of the table 9.