PURPOSE: To carry out fine cutting in the state of the center of a rotary saw coinciding with the center of a rectangular steel pipe by mounting a jig plate to one of a movable body mouthpiece and a body mouthpiece forming a stock vice, and mounting a rectangular steel pipe fixture to the other.

CONSTITUTION: A body 3 is disposed at one end part on a base 2, forming a stock vice 1, through a dial plate 4 in such a way as to be adjustable in the angle, and a mouthpiece 5 is formed at the tip face of the body 3. A movable body 6 corresponding to the body 3 is disposed at the other end part on the base 2, and a mouthpiece 7 is formed also at the tip face of the movable body 6. In this case, a fixture 11 is mounted to the mouthpiece 7 of the movable body 6, and a fixture 15 added to the outer surface, along the longitudinal direction of a jig plate 12 is brought into contact with the angular part of a rectangular steel pipe 9a. The movable body 6 is then moved toward the body 3 by feed screw mechanism 8, and the rectangular steel pipe 9a is held on a diagonal line between the mouthpiece 5 of the body 3 and the fixture 15 and then cut by a rotary saw.