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. A pair of second protrusions are formed and disposed symmetrically on both sides of the diagonal line so that the corner bottom surface is surrounded by the downward inclined surface, the first protrusion, and the second protrusions, and the corner bottom surface has the smallest height and the seating surface has the largest height along the diagonal line.