A cutting insert (130, 230), comprising: a top surface (131, 231); a bottom surface (132, 232); first to fourth side surfaces (133a, 133b, 133c, 133d, 233a, 233d) extending between the top surface and the bottom surface; and a screw hole passing through the top surface and the bottom surface, wherein the top surface includes a mounting surface (131a, 231a) comprising a flat surface, the bottom surface includes a mounting surface comprising a flat surface, and the top surface and the bottom surface are rotationally symmetric with each other, wherein a cutting edge is formed at a junction between the top surface and the first and second side surfaces (133a, 133b), the first and second side surfaces being adjacent to each other, and at a junction between the bottom surface and the third and fourth surfaces (133c, 133d), the third and fourth side surfaces being adjacent to each other, wherein a non-cutting edge is formed at a junction between the top surface and the third and fourth side surfaces (133c, 133d) and at a junction between the bottom surface and the first and second side surfaces (133a, 133b), wherein the first and second side surfaces form an acute angle with a plane extending from the mounting surface of the top surface, and the third and fourth side surfaces form an obtuse angle with the plane extending from the mounting surface of the top surface, wherein the cutting edge comprises a first cutting edge located at the top surface and a second cutting edge located at the bottom surface, wherein the first cutting edge is formed at the junction between the top surface and the first and second side surfaces (133a, 133b) and the second cutting edge is formed at the junction between the bottom surface and the third and fourth side surfaces (133c, 133d), wherein the first cutting edge comprises a first main cutting edge (134a, 234a) formed at the junction between the top surface and the first side surface (133a, 233a), wherein the second cutting edge comprises a second main cutting edge formed at the junction between the bottom surface and the fourth side surface (133d, 233d), wherein the top surface and the bottom surface are rotationally symmetric with respect to a central line parallel to a line that extends through centers of connection edges formed between the cutting edge and the non-cutting edge, and wherein the central line extends through a center of a corner located between the adjacent second and third side surfaces (133b, 133c) and a center of a corner located between the adjacent first and fourth side surfaces (133a, 133d), characterised in that a distance from the mounting surface of the bottom surface to the cutting edge located at the top surface is greater than a distance from the mounting surface of the bottom surface to the mounting surface of the top surface. a top surface (131, 231); a bottom surface (132, 232); first to fourth side surfaces (133a, 133b, 133c, 133d, 233a, 233d) extending between the top surface and the bottom surface; and a screw hole passing through the top surface and the bottom surface, wherein the top surface includes a mounting surface (131a, 231a) comprising a flat surface, the bottom surface includes a mounting surface comprising a flat surface, and the top surface and the bottom surface are rotationally symmetric with each other, wherein a cutting edge is formed at a junction between the top surface and the first and second side surfaces (133a, 133b), the first and second side surfaces being adjacent to each other, and at a junction between the bottom surface and the third and fourth surfaces (133c, 133d), the third and fourth side surfaces being adjacent to each other, wherein a non-cutting edge is formed at a junction between the top surface and the third and fourth side surfaces (133c, 133d) and at a junction between the bottom surface and the first and second side surfaces (133a, 133b), wherein the first and second side surfaces form an acute angle with a plane extending from the mounting surface of the top surface, and the third and fourth side surfaces form an obtuse angle with the plane extending from the mounting surface of the top surface, wherein the cutting edge comprises a first cutting edge located at the top surface and a second cutting edge located at the bottom surface, wherein the first cutting edge is formed at the junction between the top surface and the first and second side surfaces (133a, 133b) and the second cutting edge is formed at the junction between the bottom surface and the third and fourth side surfaces (133c, 133d), wherein the first cutting edge comprises a first main cutting edge (134a, 234a) formed at the junction between the top surface and the first side surface (133a, 233a), wherein the second cutting edge comprises a second main cutting edge formed at the junction between the bottom surface and the fourth side surface (133d, 233d), wherein the top surface and the bottom surface are rotationally symmetric with respect to a central line parallel to a line that extends through centers of connection edges formed between the cutting edge and the non-cutting edge, and wherein the central line extends through a center of a corner located between the adjacent second and third side surfaces (133b, 133c) and a center of a corner located between the adjacent first and fourth side surfaces (133a, 133d), characterised in that a distance from the mounting surface of the bottom surface to the cutting edge located at the top surface is greater than a distance from the mounting surface of the bottom surface to the mounting surface of the top surface.