To provide a hard ground excavation method capable of shortening an excavation work period when excavating a pile hole in a hard ground consisting of a rock bed or boulder stones.

A hard ground a consisting of a rock bed or boulder stones is excavated down by a rotating casing, and in parallel with the excavation, a down-the-hole hammer device 2 is installed in the inside of the rotating casing 1 in a vertically movable manner and in a state that the rotation thereof is relatively restricted. The down-the-hole hammer device 2 rotating together with the casing 1 around the center of rotation of the casing 1 makes a small hole b for inducing cracking by dropping a rock crushing bar 6 on the hard ground a in the inside of the casing 1, and the hard ground a in the inside of the casing 1 is crushed by dropping the rock crushing bar 6 to a size allowing a hammer grab 5 to grip crushed materials.