To carry out a drilling work in concentric state by correcting eccentricity of a drill from the lower hole of a work so as to set up concentric state in the initial period of work even if the longitudinal shaft center lines of the drill and the lower hole of the work are arranged in eccentricity to some extent.

A mobile jig 1 is placed in its centering arrangement by being elastically pressed at a specific position inside of the mobile jig slide guide hole 2B by means of a centering member C. Then, a gap is formed between the outside surface 1C of the mobile jig 1 and the inside surface 2D of the mobile jig slide guide hole 2B, and furthermore, a mobile jig rotation suppressing means formed by an engagingly locking flange part and an engagingly locking groove part is also arranged with a gap between themselves. At least one part of the conical surface 55A of a drill 55 attached to a drilling machine P is arranged by facing to the inside of the projecting surface of a lower hole 53A. In such a state that the conical surface 55A of the drill 55 has completely progressed into the lower hole 53A, the mobile jig 1 is moved according to its eccentricity so that the longitudinal shaft enter line of the drill 55 and that of the lower hole 53A are coaxially maintained automatically.