A cutting insert has a substantially polygonal shape, and a groove formed in a corner portion of one polygonal surface, and at an intersecting portion between a leading cutting edge and a peripheral cutting edge formed at intersecting portions of side surfaces adjacent to each other across the corner portion, a minor cutting edge is formed. Of the groove, in an area associated with the leading cutting edge and the minor cutting edge, a rake face including a downward inclined surface is formed. A first angle between a direction of maximum inclination of the downward inclined surface and the leading cutting edge is larger than a second angle between the direction of maximum inclination and the minor cutting edge.