A cutting insert has a first cutting edge provided with a chip breaker suitable for cutting an outermost region of a hole. The cutting insert has a substantially polygonal upper face, a substantially polygonal lower face, and a plurality of side faces extending between the upper face and the lower face. The plurality of side faces include a main side face constituting a flank of a linear cutting edge and a sub side face constituting a flank of a corner cutting edge; the main side face has a relief angle of 0°, the sub side face has a relief angle larger than 0°. A second cutting edge is further provided at an intersection portion between the main side face and the lower face.