The hole drilling process is for drilling a hole on a supported tube sheet by using a first drilling tool that can drill a hole with a diameter smaller than that of a mounting hole that is to become a target hole, and a second drilling tool that can drill a hole with a diameter equal to the diameter of the target hole, the process including: a temporary hole drilling step in which a temporary hole with a predetermined depth set beforehand is drilled by moving the first drilling tool to a drilled position set beforehand; a hole measuring step in which a distance (S) between an existing mounting hole and the temporary hole; a drilled position correcting step in which the drilled position is corrected based upon the distance (S) between the mounting hole and the temporary hole.