To detect the breakage of a tool in a short time by eliminating the transfer of a tool length to a numerical controller.

A tool breakage detecting device 12 mounted on and used for a machining center controlled by a numerical controller 6 has a communication line 26 connected to the numerical controller 6 for a machine tool, a tool length measuring means to measure the tool lengths L of a tool before and after mounted on the spindle of the machine tool and used for machining, a breakage judgement part 20 to compute a difference between the tool lengths before and after machining from the tool lengths measured by the tool length measuring means, before and after used for machining, and to output a breakage signal S4, if the difference between the tool lengths is equal to a preset value or more, and a communication control part 21 to output the breakage signal from the breakage judgement part 20 to the communication line 26.