To provide a non-circular excavating method for excavating a hole having a regular polygonal cross section, without marring the advantage of rotary movement.

There is provided a method for excavating a hole H in a shape of a regular polygon having N apex angles. In this method, an excavating means T is employed which is formed like a regular polygon having N-1 apex angles, with the polygon inscribed in the regular polygon having N apex angles. The excavating means T is rotated along an orbit circle R having a prescribed radius on an excavation center O, at a prescribed rotational speed, and at the same time, rotated about a center G of the excavating means at a prescribed rotational speed, to thereby excavate the hole. In place of the excavating means T which is formed like the regular polygon with N-1 apex angles, a jet may be employed. Alternatively, an excavating means formed like a regular polygon having N+1 apex angles or a drill means can also be used.