A cutting insert comprising: an insert main body which has a rake face and a flank face; and a cutting edge which is formed at a ridge section between the rake face and the flank face in the insert main body and provided with a corner section which forms a convex arc shape when seen in a planar view from the direction facing the rake face and a pair of linear sections which are in contact with the corner section at both ends of the corner section and extend linearly, wherein the insert main body is formed symmetrical with respect to a bisector of the corner section when seen in the planar view, the cutting edge is provided with a first region along the corner section, a second region along the linear section and a third region positioned between the first region and the second region, when seen in the planar view, and wherein an inclination face which is inclined toward a face on a side opposite to the rake face of the insert main body with moving toward the inside of the rake face from the cutting edge on a cross-section orthogonal to the cutting edge is formed on the rake face from the first region, the second region to the third region, characterized in that an inclination angle of the inclination face in the third region is smaller than an inclination angle of at least in one of the first region and the second region, a cross-sectional area of the insert main body on a cross-section orthogonal to the cutting edge in a range of width which is a radius R (mm) of the corner section perpendicular to the cutting edge toward the inside of the rake face when seen in the planar view is largest in the third region and smallest in the second region, of the first, the second and the third regions, and a cross-sectional area of the insert main body in the first region is between the largest cross-sectional area in the third region and the smallest cross-sectional area in the second region in terms of size. an insert main body which has a rake face and a flank face; and a cutting edge which is formed at a ridge section between the rake face and the flank face in the insert main body and provided with a corner section which forms a convex arc shape when seen in a planar view from the direction facing the rake face and a pair of linear sections which are in contact with the corner section at both ends of the corner section and extend linearly, wherein the insert main body is formed symmetrical with respect to a bisector of the corner section when seen in the planar view, the cutting edge is provided with a first region along the corner section, a second region along the linear section and a third region positioned between the first region and the second region, when seen in the planar view, and wherein an inclination face which is inclined toward a face on a side opposite to the rake face of the insert main body with moving toward the inside of the rake face from the cutting edge on a cross-section orthogonal to the cutting edge is formed on the rake face from the first region, the second region to the third region, an inclination angle of the inclination face in the third region is smaller than an inclination angle of at least in one of the first region and the second region, a cross-sectional area of the insert main body on a cross-section orthogonal to the cutting edge in a range of width which is a radius R (mm) of the corner section perpendicular to the cutting edge toward the inside of the rake face when seen in the planar view is largest in the third region and smallest in the second region, of the first, the second and the third regions, and a cross-sectional area of the insert main body in the first region is between the largest cross-sectional area in the third region and the smallest cross-sectional area in the second region in terms of size.