A cutting insert has: first sub cutting edges at positions corresponding to first corner portions of a polygonal top surface; second sub cutting edges at positions corresponding to second corner portions; and main cutting edges on side portions. In a side view, each of the main cutting edges has a first cutting edge adjacent to the first sub cutting edge, a second cutting edge adjacent to the second sub cutting edge, and a third cutting edge between the first cutting edge and the second cutting edge. Each of the main cutting edges inclines such that the height thereof decreases from a portion adjacent to the first sub cutting edge to a portion adjacent to the second sub cutting edge. An inclination angle of the third cutting edge is greater than inclination angles of the first cutting edge and the second cutting edge.