PURPOSE: To make a hole angular part machinable into an acute point (pin angle) quickly without entailing any crack by installing two chisels, being opposed to each other as separating in the axial direction, in a columnar shank.

CONSTITUTION: Respective cutting edges 4a and 5a of two chisels 4 and 5 are bent into a rectangular chevroned form on a surface orthogonal with a shaft center line of a shank 1, while they are opposed in a point symmetrical manner to a shaft center of this shank 1, thereby forming a square or rectangular four- corner part by means of two sides od each of the cutting edges 4a and 5a. An angular part machining tool A is positioned aright so as to make the two sides of these cutting edges 4a and 5a become parallel with a horizontal side 11a and a vertical side 11b of a rectangular hole 11 of a work (window frame) 10, and then it is attached to a chuck 9 of a working machine via an upper part of the shank 1. In this state intact, the angular part machining tool A is traveled in both X and Y axial directions in the hole 11 of the work 10, through which each angular part 11c of the hole is machined.