An object is to provide a cutting insert for a milling cutter with which efficiency in machining can be increased. The cutting insert includes two polygonal main surfaces facing each other and side surfaces each connected to a corresponding side of each of the main surfaces. The side surfaces include a major side surface through which an attachment hole extends, a first minor side surface perpendicular to the major side surface, and a second minor side surface disposed between the first minor side surface and the major side surface. A ridge between each of the main surfaces and the major side surface forms a major cutting edge, and a ridge between each of the main surfaces and the second minor side surface forms a minor cutting edge. An angle between the major cutting edge and the minor cutting edge is 145 or larger.

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 cutting insert according to an embodiment of the present invention is a double-sided cutting insert comprising an upper surface, a lower surface, a side surface, a cutting edge, and a mounting hole penetrating the upper surface and the lower surface, in which the upper surface is provided with a single upper surface boss portion that surrounds the mounting hole and is flat, the cutting edge includes a main cutting edge, a corner cutting edge extending from the main cutting edge, a sub-cutting edge extending from the corner cutting edge, and a side portion extending from the sub-cutting edge, a first main cutting edge flank surface extending from the main cutting edge forms an acute angle with an extension line of the upper surface boss portion, a sub-cutting edge chip breaker surface extending from the sub-cutting edge is at a gradually increasing angle with the extension line of the upper surface boss portion, as the sub-cutting edge chip breaker surface is distanced away from the corner cutting edge, and when the cutting insert is viewed into the upper surface, a side portion surface extending from the side portion is concavely recessed.