A cutting insert comprising: an insert main body having a rake face; a cutting edge formed at a circumferential edge of the rake face of the insert main body to have a corner edge and two side edges arranged on both sides of the corner edge; a protrusion portion formed on the rake face inside the corner edge so as to protrude, with a space kept between the cutting edges; and a pair of convex portions which are formed from a wall surface of the protrusion portion to the rake face so as to extend out to the side edges on both sides divided by a bisector of the corner edge; wherein each of the pair of convex portions is provided with a first wall surface and a second wall surface, the first wall surfaces extending so as to separate gradually from each other toward the cutting edge from the wall surface of the protrusion portion, each first wall surface facing the corner edge of the cutting edge, the second wall surface contacting with the first wall surface and facing the side edge continuing from an end of the corner edge, the first and the second wall surfaces (7A, 7B) are inclined surfaces inclining toward the rake face as they extend away from a ridge line between both wall surfaces, on the rake face closer to the corner edge than the pair of convex portions, a recess which has a rear wall portion extending so as to cross the bisector in the middle between the corner edge and the convex portion and which is recessed with respect to the corner edge is formed wherein the rake face between the first wall surface of the pair of convex portions is formed as a flat rake face perpendicular to the insert main body's thickness direction, characterized in that the ridge line is in contact with the wall surface of the protrusion portion at a position lower than the top portion of the protrusion portion closer to the corner edge. an insert main body having a rake face; a cutting edge formed at a circumferential edge of the rake face of the insert main body to have a corner edge and two side edges arranged on both sides of the corner edge; a protrusion portion formed on the rake face inside the corner edge so as to protrude, with a space kept between the cutting edges; and a pair of convex portions which are formed from a wall surface of the protrusion portion to the rake face so as to extend out to the side edges on both sides divided by a bisector of the corner edge; wherein each of the pair of convex portions is provided with a first wall surface and a second wall surface, the first wall surfaces extending so as to separate gradually from each other toward the cutting edge from the wall surface of the protrusion portion, each first wall surface facing the corner edge of the cutting edge, the second wall surface contacting with the first wall surface and facing the side edge continuing from an end of the corner edge, the first and the second wall surfaces (7A, 7B) are inclined surfaces inclining toward the rake face as they extend away from a ridge line between both wall surfaces, on the rake face closer to the corner edge than the pair of convex portions, a recess which has a rear wall portion extending so as to cross the bisector in the middle between the corner edge and the convex portion and which is recessed with respect to the corner edge is formed wherein the rake face between the first wall surface of the pair of convex portions is formed as a flat rake face perpendicular to the insert main body's thickness direction, characterized in that the ridge line is in contact with the wall surface of the protrusion portion at a position lower than the top portion of the protrusion portion closer to the corner edge. a protrusion portion formed on the rake face inside the corner edge so as to protrude, with a space kept between the cutting edges; and a pair of convex portions which are formed from a wall surface of the protrusion portion to the rake face so as to extend out to the side edges on both sides divided by a bisector of the corner edge; wherein each of the pair of convex portions is provided with a first wall surface and a second wall surface, the first wall surfaces extending so as to separate gradually from each other toward the cutting edge from the wall surface of the protrusion portion, each first wall surface facing the corner edge of the cutting edge, the second wall surface contacting with the first wall surface and facing the side edge continuing from an end of the corner edge, the first and the second wall surfaces (7A, 7B) are inclined surfaces inclining toward the rake face as they extend away from a ridge line between both wall surfaces, on the rake face closer to the corner edge than the pair of convex portions, a recess which has a rear wall portion extending so as to cross the bisector in the middle between the corner edge and the convex portion and which is recessed with respect to the corner edge is formed wherein the rake face between the first wall surface of the pair of convex portions is formed as a flat rake face perpendicular to the insert main body's thickness direction,