PURPOSE: To cut a square pipe properly without deformation.

CONSTITUTION: There is provided a cylindrical holding guide 3 which comes into contact with the outer peripheral surface of a square pipe 2 to hold the square pipe 2 and is formed with a slit 5 through which a cutter 4 is inserted. There is provided a sharp-pointed cutter 4 formed with inclined cutting blades 7a, 7a on both sides. The cutter 4 is inserted into a recessed groove 20 formed at the square pipe 2 through the slit 5 to cut a sidewall 2a forming the recessed groove 20, and cuts the other sidewalls, 2b, 2c, and 2d of the square pipe 2 while pressing them respectively against sides of the cylindrical holding guide 3 by the inclined cutting blades 7a, 7a.