To provide a technology for directly confirming the enlargement of an enlarged diameter blade by a sensor since there have been no means for confirming the enlargement of the enlarged diameter blade mounted to the lower end of an auger drill in a technology for forming an enlarged bulb in the support ground of the lower end of a pile.

The sensor 30 for directly detecting the opening/closing operation of the enlarged diameter blade of the auger drill is provided at the lower end 10 of the auger drill, and a communication cable 31 is disposed in a hollow part of an auger drill shaft. A connector 32 having a contact terminal crimped when connecting the auger drill is provided at a connection part of the auger drill shaft, and a signal of the sensor 30 can be confirmed on the ground by a display device of a voice signal or a visible signal, a recording device or the like.