PURPOSE: To construct an expanded anchor in a large cross section in a correct position in an optional area by expanding a hole with high-pressure jet water ejected from a high-pressure water ejection rod.

CONSTITUTION: Drilling is made to a specified depth with a drilling rod, and a casing in a part corresponding to an anchorage part on the front end of the drilled hole 1 is extracted. Then, a high-pressure water ejection rod 2 is inserted from the inside of a casing positioned in a free length part on the tail end of the drilled hole 1 until it reaches the front end of the anchorage part. The rod 2 is then extracted little by little while high-pressure water is ejected from the front end of the rod 2 to the surroundings with a high-pressure plunger pump, and the ground surrounding the drilled hole 1 is cut with the high-pressure water and an expanded hole 3 is made thereby. Then, the drilling rod is inserted into the drilled hole 1 until it reaches the front end of the hole, and cleaning is made inside the drilled hole 1 with mud discharged by running of water. A tension steel is inserted into the drilled hole 1 and an anchorage body is constructed with grouting 4 made under pressure either to the expanded hole 3 in the anchorage part or the drilled hole 1.