To confirm the excavating diameter of an enlarging head attached to the auger head of an excavation device and the continuity of an excavated hole wall on the ground in a simple structure.

An outer rod 8 disposed on both sides of an excavation rod 4 for rotatingly driving the auger head 7 is vertically moved relative to an excavation rod 3 along the excavation rod by a hydraulic jack 20a. The enlarging blades 73a of the enlarging head 73 which constitutes the lower end of the outer rod 8 and the pantograph attached to the excavation rod 3 are enlarged/contracted by the vertical movement. The operator can confirm the enlarged amount of the enlarging head 73 (a pair of enlarging blades 73a), i.e., the excavation diameter thereof on the basis of the relative displaced amount of the outer rod 8 to the excavation rod 3.