To enable machining for thinning hole with small dimensions without damaging the seating stability to a tool main body by making an included angle of both chip sides connected to a nose part larger than the tip end angle of the nose.

In all corner parts 13 of a chip 11, a single end part of one side surface 11C of two side surfaces 11C forming each corner part 13 is recessed from the surface to form a nose part 14 having a tip end angle αsmaller than an included angle β made between two side surfaces 11C, and further a cutting edge 14a is formed on the edge side of the tip end part of each nose part 14. Since the tip end angle and the included angle have the indepent relation geometrically, the included angle β is kept about 60°, and only the tip angle can be decreased according to the dimensions of a thinning hole. Accordingly, when the chip is mounted on the tool main body, the projection length of the nose part 14 is kept from being increased or the seating area is kept from being reduced.