A throwaway insert comprises an insert body formed in a polygonal shape, a cutting face formed by a polygonal face of the insert body, a corner cutting edge which is formed in a curved shape at a corner of the cutting face, a major cutting edge which is formed along a side edge of the cutting face that is connected to a first end of the corner cutting edge, and a concave portion which is formed, in another side edge of the cutting face that is connected to a second end of the corner cutting edge, so as to be adjacent to the second end of the corner cutting edge, and so as to be made concave toward the inside of the cutting face from a tangent of the corner cutting edge drawn at the second end of the corner cutting edge. By using the throwaway insert, cut chips are prevented from being packed in a gap between the throwaway insert and a surface being machined, and the fixing rigidity of the throwaway insert is increased.