Abstract: As viewed from a side opposing a bottom face (6) of an insert attachment seat, an angle (?) between first and second restraint wall faces (7a) and (7b) rising from the bottom face (6) is smaller than an angle (?) between two peripheral faces of a cutting insert contacting them respectively. A first peripheral face of the cutting insert contacts the first restraint wall face (7a) in a substantially parallel posture with the extending direction of the first restraint wall face (7a), while the second peripheral face contacts the second restraint wall face (7b) in an end portion of the second restraint wall face (7b).