A hexahedral-shaped cutting insert is disclosed which has an upper surface, a lower surface, a plurality of side surfaces connecting the upper surface and the lower surface and a central hole passing through the upper and lower surfaces. The lower surface has a flat mounting surface, and the upper surface is curved concavely over the whole area thereof and provided with cutting edges along the borderline to the side surfaces, and corner cutting edge portions formed at borders of two adjacent cutting edges. The cutting edges are formed such that the height is largest at corner cutting edge portions and smallest in the middle portions. A land portion, a downward inclined surface, a bottom surface, and a seating surface portion are formed from the cutting edges inwards on the upper surface. A plurality of semispherical protrusions are formed on border portions between the downward inclined surface and the bottom surface, and at least one corner cutting edge portion has a corner bottom surface surrounded by an arc belt protrusion which connects two adjacent semispherical protrusions.