The present invention discloses a cutting insert for a machining tool having a structure being capable of controlling effectively chips generated on a work-piece. A cutting insert according to the present invention includes 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 side surfaces lying on an imaginary polygon. In the cutting insert, a flat seating surface is formed on a highest portion of the upper surface, a plurality of side cutting edge portions are formed at border portions of the upper surface and the side surfaces, each side cutting edge portion has a side cutting edge formed at an upper end thereof, a corner cutting edge portion is formed at a border portion of two adjacent side cutting edge portions, the corner cutting edge portion has a cutting edge formed at a front end thereof, a land portion, a downward inclined surface, a corner bottom surface, a first upward inclined surface, a first protrusion, a second upward inclined surface, a second protrusion, an upward grooved surface and the seating surface are sequentially formed sequentially on the corner cutting edge portion along a diagonal line directed from the corner cutting edge to the central hole, and the corner bottom surface has the smallest height along the diagonal line.