Abstract: A cutting insert has an upper surface, a lower surface, a plurality of side surfaces connecting the upper surface and the lower surface and a central hole formed at a central portion thereof. The cutting insert has a flat seating surface formed on a highest level of the upper surface, a plurality of side cutting edge portions formed at intersections of the upper surface and the side surfaces, and a corner cutting edge portion formed at an intersection of two adjacent side cutting edge portions and having a cutting edge formed at a front end thereof. A land portion, a downward inclined surface, a corner bottom surface, a first protrusion, an upward inclined surface and the seating surface are formed on the corner cutting edge portion sequentially along a diagonal line directed from the cutting edge to the central hole.