To detect retreating of a movable board by a simple means by using a detector head for forward energizing the movable board and supporting it, on which a detector capable of being inserted into the machining hole of a work piece is attached, in a device for detecting the breakage of a tool to a machining hole caused by retreating of the movable board in relation to a head when the head is connected to a driving unit and advanced to the specified detecting position.

An air blow-off passage 12 whose downstream end is opened to a head connecting part 4a of a driving unit and a pressure detecting means 13 for detecting air pressure of the passage are provided to confirm the connection of a detector head 8 to the driving unit. An air inflow passage 16 to be connected to the air blow-off passage 12 by the connection to the driving unit is provided on the detector head 8. A limit valve 17 for releasing the air inflow passage 16 to the atmosphere in opening of the valve is provided on one of the detector head 8 and the movable board 15, a dog 18 is provided on the other one, and the limit valve 17 is opened by the dog 18 after the movable, board 15 is retreated in relation to the detector head 8. Pressure drop by the valve opening is detected by the pressure detecting means 13.