A cutting insert for high-efficiency cutting is disclosed. The cutting insert includes an upper surface (110a), a lower surface (HOb), side surfaces (114) and corner side surfaces (116), which connect the side surfaces to each other. A circular opening is formed through the central portions of the upper and lower surfaces. In the cutting insert, cutting edges are formed by respective junctions between one of the upper and lower surfaces and the side surfaces and between one of the upper and lower surfaces and the corner side surfaces. The cutting edges comprise three corner cutting edges (122), six major cutting edges (124), which extend from opposite ends of the corner cutting edges towards the neighbor corner cutting edges, and three minor cutting edges (126), which are connected between the major cutting edges. Furthermore, the cutting edges may be formed on both the upper surface and the lower surface.