The boring tool is designed to gain high clamp power by soft releasing and tightening of cartridge using a mounting screw with common and thin pitch screw. The boring tool includes : the first and the second cartridges (2) (2a) mounted to the first arm screw section (5); a V-shaped guide groove (10) mounted to the first and the second cartridges (2) (2a) via the second screw groove (11); and a set screw (12) mounted on a slope at lower position than the V-shaped guide groove (10).
