To versatilely apply a circular hole boring tool to various power tools.

This circular hole boring tool 1 for boring a circular hole in a boring object, is provided with a bit holder 3 being concentrically installable on a driving shaft of the power tools so as to be co-rotatable and concentrically fixing a bit 2 on the tip, a cover member 4 installed on this bit holder 3 so as to be relatively rotatable around the axis and a radial rod 5 attached in the direction for crossing the bit 2 in this cover member 4 so as to be capable of varying a projecting quantity. The cover member 4 is set in a dimension so that the bit 2 projects outside from the edge. The radial rod 5 has a tip pointed central rod 52 formed by bending one side part at an almost right angle.