PURPOSE: To cut a square pipe properly without deformation.

CONSTITUTION: A cylindrical holding guide 3 is made to come into contact with the outer peripheral surface of a square pipe 2 to hold the square pipe 2. The first cutter 4 provided with a sharp-pointed inclined cutting blade 4a whose rearer side goes more outward from the cylindrical holding guide 3 is inserted through a slit 5 formed at the cylindrical holding guide 3 and cuts the one sidewall 2a of the square pipe 2 while pressing it outward of the cylindrical holding guide 3. The second cutter 7 is inserted from a cut line 6 generated on the one sidewall 2a in the approximately orthogonal direction of the first cutter 4, and cuts the other sidewalls 2b, 2c, and 2d of the square pipe 2 while pressing them respectively outward from the cylindrical holding guide 3 by the inclined cutting blades 7a, 7a formed on both the sides of the second cutter 7.