A cutting insert, comprising: an upper surface with a polygonal shape which has a side part, and a first corner part and a second corner part each being adjacent to the side part; a lower surface with a polygonal shape which corresponds to the upper surface; a side surface disposed between the lower surface and the upper surface; a first corner cutting edge that is an intersection of the upper surface and the side surface and is located on the first corner part; a second corner cutting edge that is an intersection of the upper surface and the side surface and is located on the second corner part; a flat cutting edge that is an intersection of the upper surface and the side surface and is disposed on the side part; a major cutting edge that is an intersection of the upper surface and the side surface and is disposed between the second corner cutting edge and the flat cutting edge on the side part; and a connecting edge that is an intersection of the upper surface and the side surface and is disposed between the flat cutting edge and the major cutting edge, wherein the flat cutting edge and the major cutting edge have a straight line shape, and the connecting edge has an outwardly protruding curvilinear shape in a top view, characterized in that the major cutting edge has an upwardly protruding shape, and comprises an upwardly protruding curvilinear portion, a first straight line portion extending from the curvilinear portion toward the connecting edge, and a second straight line portion extending from the curvilinear portion toward the second corner cutting edge in a side view, wherein the flat cutting edge, the connecting edge and the first straight line portion are located on a straight line in the side view. an upper surface with a polygonal shape which has a side part, and a first corner part and a second corner part each being adjacent to the side part; a lower surface with a polygonal shape which corresponds to the upper surface; a side surface disposed between the lower surface and the upper surface; a first corner cutting edge that is an intersection of the upper surface and the side surface and is located on the first corner part; a second corner cutting edge that is an intersection of the upper surface and the side surface and is located on the second corner part; a flat cutting edge that is an intersection of the upper surface and the side surface and is disposed on the side part; a major cutting edge that is an intersection of the upper surface and the side surface and is disposed between the second corner cutting edge and the flat cutting edge on the side part; and a connecting edge that is an intersection of the upper surface and the side surface and is disposed between the flat cutting edge and the major cutting edge, wherein the flat cutting edge and the major cutting edge have a straight line shape, and the connecting edge has an outwardly protruding curvilinear shape in a top view, characterized in that the major cutting edge has an upwardly protruding shape, and comprises an upwardly protruding curvilinear portion, a first straight line portion extending from the curvilinear portion toward the connecting edge, and a second straight line portion extending from the curvilinear portion toward the second corner cutting edge in a side view, wherein the flat cutting edge, the connecting edge and the first straight line portion are located on a straight line in the side view.