To obtain a clear cut section and to shorten cutting time.

In a condition of a metal-made square cylinder 13 inserted and supported on square opening parts 3, 3 of fixed cutting tooth shape 1, 1, movable cutting tooth shapes 2A, 2B are moved in a direction of approaching each other, the tip end cutting teeth 4A, 4B in the movable cutting tooth shapes 2A, 2B are made to bite in diagonal parts 13a, 13b of the square cylinder 13, thereafter upper/lower horizontal surfaces 13e, 13f of the square cylinder 13 are cut by the tip end cutting teeth 4A, 4B. Right/left vertical surfaces 13c, 13d of the square cylinder 13 are cut by intermediate cutting teeth 6A, 6B, and at the time when the tip end cutting teeth 4A, 4B vertically cross with rear end cutting teeth 5B, 5A, cutting of the square cylinder 13 is completed.