A cutting insert of the present invention has a body, auxiliary cutting edges of a convex curve shape and main cutting edges. The body has a top surface, a bottom surface, side surfaces connecting the top and bottom surfaces, and corner surfaces connecting the adjacent side surfaces. The auxiliary cutting edges are formed at intersections between the top and bottom surfaces and the side surfaces. The main cutting edges are formed at intersections between the top and bottom surfaces and the corner surfaces. The main cutting edge in the top surface extends from an end of an adjacent auxiliary cutting edge to its center in a direction toward the bottom surface and from the center to an end of another adjacent auxiliary cutting edge in a direction away from the bottom surface.