To form a strongly reinforced improved zone in bedrock in a short time by excavating a tunnel after the bedrock is reinforced by arranging friction bolts to expand in the radial direction by injecting high pressure water, in a hole arranged in the obliquely forward direction of tunnel working face bedrock.

In tubular expansive bolts (friction bolts) 1, both ends of an expansive pipe having a hollow structure composed of soft steel or the like are fixed by a reinforced fixing pipe, and a high pressure water introducing pipe 4 is installed in the reinforced fixing pipe by an end plate. The friction bolts 1 are inserted into a long hole 53 from a working face in a top end part 52 of the working face of a tunnel, and an installing plate 7 is installed on base rock by a nut 8 inserted into the reinforced fixing pipe, and high pressure water is introduced into the expansive pipe 2 through the high pressure water introducing pipe 4 from a pump 9. The expansive pipe 2 expands by this high pressure water, and contacts with an inner wall of the long hole 53, and when it is balanced with reaction of the base rock, the expansion is stopped, and the friction bolts 1 are fixed to the long hole 53.