A cutting insert may include a first surface, a second surface and a lateral surface. The first surface may have a polygonal shape and may include a first corner, a second corner, a third corner, a first side and a second side. The lateral surface may include a first lateral surface and a second lateral surface. At least a part of the first lateral surface may be located between the first side and the second surface. At least a part of the second lateral surface may be located between the second side and the second surface. The first lateral surface may be located further outside than the first side in a front view of the first surface. The first lateral surface may include a first region located closer to the second lateral surface as coming closer to the second surface.

A cutting insert includes an insert body having a polygonal plate shape. Rake faces are formed on polygonal surfaces of the insert body, and flank faces are formed on lateral faces of the insert body arranged around the polygonal surfaces. Main cutting edges are formed on side ridges of the polygonal surfaces at intersecting ridgelines between the rake faces and the flank faces. Each main cutting edge extends between two corners adjacent to each other in a circumferential direction of the polygonal surface among corners of the polygonal surface. When seen from a direction opposing the rake face, one end portion of the main cutting edge that stretches to one corner of the two corners has a convexly curved shape, and the other end portion of the main cutting edge that stretches to the other corner of the two corners has a concavely curved shape.

A substantially parallelogramatic insert having a downward inclined main cutting edge and an auxiliary cutting edge in part of a long-side ridgeline and a short-side ridgeline adjacent to a pair of corner cutting edges of an upper surface; the main cutting edge being constituted by first to third straight cutting edges; the first to third straight cutting edges being connected in a bent-line shape with inward obtuse crossing angles when viewed from above, such that the second straight cutting edge is positioned outermost; and cutting edge angles .sub.1, .sub.2, .sub.3 between the rake faces and flanks of the first to third straight cutting edges meeting the relation of .sub.1>.sub.2>.sub.3.