To enable the stable use of a back face under pressure in the same manner as when a front face is used by notching a square pole-like body arm for introducing and anchoring an angle adjusting plate or a body arm fixing plate, and causing an antiskid rubber to stick on account of its dovetail suction design fabrication.

A square pole-like body arm 1 is notched for introducing and anchoring an angle adjusting plate 2 or a body arm fixing plate 4, and an antiskid rubber 5 is fitted by fabricating it in a dovetail suction fashion. The body arm fixing plate 4 is fixed with a screw 1, and the angle adjusting plate 2 is fixed in another notch with a rivet 6. A guide rule 3 is set in such a way that its center meets the center of the body arm 1, and a grade graduation 11 is set on the return grade of a roof grade and the center of the body arm 1 is pressed against a rafter to be cut. Further, a motor-driven circular saw with a blade and a bench which are previously set as a specified angle, is pressed against the guide rule 3 and cuts the rafter in its sliding action. Next, the rule is reversed and the rafter is cut likewise. Even when the back face of the rule is used, it can be stably used without a gap as is the case with the front face.