A cutting insert, comprising: an upper surface comprising a plurality of side parts; a lower surface comprising a plurality of side parts; a side surface located between the upper surface and the lower surface; an upper cutting edge being located along one of the side parts on the upper surface and comprising a first upper cutting edge and a second upper cutting edge; a lower cutting edge being located along one of the side parts on the lower surface and comprising a first lower cutting edge and a second lower cutting edge; and a through hole extending between the upper surface and the lower surface, wherein the first lower cutting edge is located below the second upper cutting edge and the second lower cutting edge is located below the first upper cutting edge, such that, when the cutting insert is reversed, the first lower cutting edge can take the position of the first upper cutting edge and the second lower cutting edge can take the position of the second upper cutting edge, whereby the lower cutting edge is usable in the same manner as the upper cutting edge by turning the insert upside down, characterised in that the side surface comprises a first surface extending downward from the second upper cutting edge, a second surface extending upward from the first lower cutting edge, a third surface extending downward from the first upper cutting edge, and a fourth surface extending upward from the second lower cutting edge, wherein the first surface and the fourth surface are parallel to the central axis, wherein the second surface is inclined so as to further depart from the central axis than the first surface as going from the lower surface toward the upper surface, wherein the third surface is inclined so as to further depart from the central axis than the fourth surface as going from the upper surface toward the lower surface, and wherein each of the first surface, the second surface, the third surface and the fourth surface is a flat surface. an upper surface comprising a plurality of side parts; a lower surface comprising a plurality of side parts; a side surface located between the upper surface and the lower surface; an upper cutting edge being located along one of the side parts on the upper surface and comprising a first upper cutting edge and a second upper cutting edge; a lower cutting edge being located along one of the side parts on the lower surface and comprising a first lower cutting edge and a second lower cutting edge; and a through hole extending between the upper surface and the lower surface, wherein the first lower cutting edge is located below the second upper cutting edge and the second lower cutting edge is located below the first upper cutting edge, such that, when the cutting insert is reversed, the first lower cutting edge can take the position of the first upper cutting edge and the second lower cutting edge can take the position of the second upper cutting edge, whereby the lower cutting edge is usable in the same manner as the upper cutting edge by turning the insert upside down, characterised in that the side surface comprises a first surface extending downward from the second upper cutting edge, a second surface extending upward from the first lower cutting edge, a third surface extending downward from the first upper cutting edge, and a fourth surface extending upward from the second lower cutting edge, wherein the first surface and the fourth surface are parallel to the central axis, wherein the second surface is inclined so as to further depart from the central axis than the first surface as going from the lower surface toward the upper surface, wherein the third surface is inclined so as to further depart from the central axis than the fourth surface as going from the upper surface toward the lower surface, and wherein each of the first surface, the second surface, the third surface and the fourth surface is a flat surface.