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 insert is turned upside down by reversing the central axis, the first upper cutting edge and the first lower cutting edge overlap and the second upper cutting edge and the second lower cutting edge overlap, wherein the first upper cutting edge and the second upper cutting edge are individually in a straight line form in a top view, wherein the first lower cutting edge and the second lower cutting edge are individually in a straight line form in a bottom view, wherein the upper cutting edge is in a protruded shape having an obtuse angle formed by the first upper cutting edge and the second upper cutting edge in the top view, and wherein the lower cutting edge is in a protruded shape having an obtuse angle formed by the first lower cutting edge and the second lower cutting edge in the bottom view, 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 a central axis of the through hole, wherein the second surface is inclined so as to depart from the central axis as going from the lower surface toward the upper surface, wherein the third surface is inclined so as to depart from the central axis as going from the upper surface toward the lower 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 insert is turned upside down by reversing the central axis, the first upper cutting edge and the first lower cutting edge overlap and the second upper cutting edge and the second lower cutting edge overlap, wherein the first upper cutting edge and the second upper cutting edge are individually in a straight line form in a top view, wherein the first lower cutting edge and the second lower cutting edge are individually in a straight line form in a bottom view, wherein the upper cutting edge is in a protruded shape having an obtuse angle formed by the first upper cutting edge and the second upper cutting edge in the top view, and wherein the lower cutting edge is in a protruded shape having an obtuse angle formed by the first lower cutting edge and the second lower cutting edge in the bottom view, 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 a central axis of the through hole, wherein the second surface is inclined so as to depart from the central axis as going from the lower surface toward the upper surface, wherein the third surface is inclined so as to depart from the central axis as going from the upper surface toward the lower surface.