A cutting insert removably attachable to a mounting seat of a cutting tool body, comprising: a first rake face formed on a first surface and a second rake face formed on a second surface, the first surface being a top surface of the cutting insert and the second surface being a bottom surface of the cutting insert; a first flank and a second flank formed on a side surface extending between the first surface and the second surface; a first cutting edge formed at an intersection portion between the first rake face and the first flank, and a second cutting edge formed at an intersection portion between the second rake face and the second flank, the cutting insert being rotationally symmetric by 180 degrees with respect to a center axis of the cutting insert which is defined to be perpendicular to a reference surface of the first surface and a reference surface of the second surface; and the first and second cutting edges (13A, 13B) being rotationally symmetric by 180 degrees about a first axis which is perpendicular to the center axis; wherein the first cutting edge has a corner cutting edge portion (13A_2) formed at a corner portion of the first surface, and a cutting edge portion (13A_1) which is continuous from the corner cutting edge portion (13A_2); and the second cutting edge has a corner cutting edge portion (13B_2) formed at a corner portion of the second surface, and a cutting edge portion (13B_1) which is continuous from the corner cutting edge portion (13B_2); wherein the cutting edge portion (13A_1, 13B_1) in each of the first and second cutting edges (13A, 13B) is inclined in a direction away from a first imaginary plane (PL3, PL4) perpendicular to the center axis of the cutting insert and passing through the corresponding corner cutting edge portion(13A_2, 13B_2), as it goes away from the corresponding corner cutting edge portion (13A_2, 13B_2); characterised in that the first flanks and the second flanks are formed so as to have positive clearance angles, respectively. a first rake face formed on a first surface and a second rake face formed on a second surface, the first surface being a top surface of the cutting insert and the second surface being a bottom surface of the cutting insert; a first flank and a second flank formed on a side surface extending between the first surface and the second surface; a first cutting edge formed at an intersection portion between the first rake face and the first flank, and a second cutting edge formed at an intersection portion between the second rake face and the second flank, the cutting insert being rotationally symmetric by 180 degrees with respect to a center axis of the cutting insert which is defined to be perpendicular to a reference surface of the first surface and a reference surface of the second surface; and the first and second cutting edges (13A, 13B) being rotationally symmetric by 180 degrees about a first axis which is perpendicular to the center axis; wherein the first cutting edge has a corner cutting edge portion (13A_2) formed at a corner portion of the first surface, and a cutting edge portion (13A_1) which is continuous from the corner cutting edge portion (13A_2); and the second cutting edge has a corner cutting edge portion (13B_2) formed at a corner portion of the second surface, and a cutting edge portion (13B_1) which is continuous from the corner cutting edge portion (13B_2); wherein the cutting edge portion (13A_1, 13B_1) in each of the first and second cutting edges (13A, 13B) is inclined in a direction away from a first imaginary plane (PL3, PL4) perpendicular to the center axis of the cutting insert and passing through the corresponding corner cutting edge portion(13A_2, 13B_2), as it goes away from the corresponding corner cutting edge portion (13A_2, 13B_2); characterised in that the first flanks and the second flanks are formed so as to have positive clearance angles, respectively.