A cutting insert, having a polygonal plate shape and comprising: an upper surface, a lower surface, a side surface located between the upper surface and the lower surface and a cutting edge formed at a ridgeline where the upper surface and the side surface intersect each other, wherein the upper surface comprises a convex shaped major part having a flat upper end surface, a first protrusion protruding from the major part toward a corner part of the upper surface, a second protrusion protruding from the first protrusion toward the corner part, and a pair of projections located between the second protrusion and the cutting edge so as to interpose between the projections a region lying between a front end of the second protrusion and the corner part, wherein a height of an upper end of the first protrusion is lower than a height of the upper end surface of the major part and is higher than a height of the cutting edge, and wherein a height of an upper end of each of the pair of projections is lower than the height of the second protrusion, characterized in that a height of an upper end of the second protrusion is equal to the height of the cutting edge, wherein a front end of the major part comprises a flat first inclined surface and a concave shaped second inclined surface nearer the corner part than the first inclined surface, wherein a front end of the first protrusion comprises a convex shaped third inclined surface and a concave shaped fourth inclined surface nearer the corner part than the third inclined surface, and wherein a front end of the second protrusion comprises a convex shaped fifth inclined surface and a concave shaped sixth inclined surface nearer the corner part than the fifth inclined surface. wherein the upper surface comprises a convex shaped major part having a flat upper end surface, a first protrusion protruding from the major part toward a corner part of the upper surface, a second protrusion protruding from the first protrusion toward the corner part, and a pair of projections located between the second protrusion and the cutting edge so as to interpose between the projections a region lying between a front end of the second protrusion and the corner part, wherein a height of an upper end of the first protrusion is lower than a height of the upper end surface of the major part and is higher than a height of the cutting edge, and wherein a height of an upper end of each of the pair of projections is lower than the height of the second protrusion, characterized in that a height of an upper end of the second protrusion is equal to the height of the cutting edge, wherein a front end of the major part comprises a flat first inclined surface and a concave shaped second inclined surface nearer the corner part than the first inclined surface, wherein a front end of the first protrusion comprises a convex shaped third inclined surface and a concave shaped fourth inclined surface nearer the corner part than the third inclined surface, and wherein a front end of the second protrusion comprises a convex shaped fifth inclined surface and a concave shaped sixth inclined surface nearer the corner part than the fifth inclined surface.