To provide an enlarged head having an enlarged blade which is forcibly opened and closed by half-rotating an excavating rod relative to the enlarged head to surely enlarge a vertical hole, and the opening and closing of which can be detected by monitoring the state of the load applied to the excavating rod when the excavating rod is half-rotated.

The enlarged head connected to the bottom end of the excavating rod includes an excavating head having a pair of pressure receiving parts which are arranged symmetrically at the upper part of a spiral blade disposed around a hollow shaft, and an acting head which has a drive shaft which is fitted to the excavating rod and to which a rotational force is transmitted, the spiral blade disposed around the intermediate portion of the drive shaft, and a pair of pressing parts disposed symmetrically at the lower part of the spiral blade in which the lower portion of the drive shaft is inserted into the hollow shaft, and to which the excavating head is rotatably attached. The pressing parts are engaged with the pressure receiving parts, and apply rotational forces thereto, and the pressing parts are engaged with the enlarged blade, and open and close the enlarged blade.