A cutting insert , comprising: a polygonal shaped upper surface comprising a pair of corner parts and a side part located between the pair of corner parts; a polygonal shaped lower surface corresponding to the upper surface ; a side surface disposed between the lower surface and the upper surface ; a pair of corner cutting edges located along an intersection of the pair of corner parts of the upper surface and the side surface ; and a major cutting edge located along an intersection of the side part of the upper surface and the side surface , wherein the major cutting edge has a downwardly dented concave shape as a whole, and comprises a first section having a downwardly dented concave curvilinear shape in a side view, and a pair of second sections being in a shape of a straight line, the pair of second sections respectively extending from the first section toward the pair of corner cutting edges and having a larger height as approaching the pair of corner cutting edges , wherein a portion of the first section which is located lowermost is closer to one of the corner cutting edges than the other of the corner cutting edges in the side view, wherein the upper surface comprises a first rake surface located along the first section of the major cutting edge , a second rake surface located along the second section of the major cutting edge , and a third rake surface located along the corner cutting edge , characterized in that a rake angle of the second rake surface is larger than a rake angle of each of the first rake surface and the third rake surface . a polygonal shaped upper surface comprising a pair of corner parts and a side part located between the pair of corner parts; a polygonal shaped lower surface corresponding to the upper surface ; a side surface disposed between the lower surface and the upper surface ; a pair of corner cutting edges located along an intersection of the pair of corner parts of the upper surface and the side surface ; and a major cutting edge located along an intersection of the side part of the upper surface and the side surface , wherein the major cutting edge has a downwardly dented concave shape as a whole, and comprises a first section having a downwardly dented concave curvilinear shape in a side view, and a pair of second sections being in a shape of a straight line, the pair of second sections respectively extending from the first section toward the pair of corner cutting edges and having a larger height as approaching the pair of corner cutting edges , a first section having a downwardly dented concave curvilinear shape in a side view, and a pair of second sections being in a shape of a straight line, the pair of second sections respectively extending from the first section toward the pair of corner cutting edges and having a larger height as approaching the pair of corner cutting edges , wherein a portion of the first section which is located lowermost is closer to one of the corner cutting edges than the other of the corner cutting edges in the side view, wherein the upper surface comprises a first rake surface located along the first section of the major cutting edge , a second rake surface located along the second section of the major cutting edge , and a third rake surface located along the corner cutting edge , characterized in that a rake angle of the second rake surface is larger than a rake angle of each of the first rake surface and the third rake surface . a first rake surface located along the first section of the major cutting edge , a second rake surface located along the second section of the major cutting edge , and a third rake surface located along the corner cutting edge , characterized in that a rake angle of the second rake surface is larger than a rake angle of each of the first rake surface and the third rake surface . <EOS>