To provide a cutting insert being favorably used for shoulder cutting of 90, having a sub cutting edge with excellent strength and elongating the life of a tool in the cutting insert forming a substantially quadrangular upper face.

This cutting insert is provided with a lower face having a substantially quadrangular shape, the upper face facing the lower face, sides, cutting edge parts formed at the upper face and the side edges, wherein the cutting edge parts have four sets of corner cutting edges located at the corner parts of the substantial quadrangle, and main cutting edges and sub cutting edges located at the both sides of the corner cutting edges. Each side face has a main flank corresponding a main cutting edge, a sub flank corresponding to a sub cutting edge, and a corner flank corresponding to a corner cutting edge. The main cutting edge is located between the corner cutting edge of the same set and the sub cutting edge of an adjacent set, and located at a higher position than the sub cutting edge of the adjacent set in the side view and inward in the top view. The corner flank has an upper corner flank located close to the corner cutting edge, and a lower corner flank located below the upper corner flank in contact with the sub flank.