A cutting insert (1, 1A), comprising: an upper surface; a lower surface; a side surface which is connected to each of the upper surface and the lower surface, and comprises a first side surface and a second side surface adjacent to each other; and a cutting edge comprising a major cutting edge located at an intersection of the upper surface and the first side surface, and a minor cutting edge located at an intersection of the upper surface and the second side surface, wherein the upper surface comprises a rake surface which is located along the major cutting edge and is inclined downward as the upper surface separates from the major cutting edge, the second side surface comprises a second upper constraining surface and a second lower constraining surface in order as the second side surface goes from the upper surface to the lower surface, wherein the second upper constraining surface is inclined inward at an inclination angle α1 with reference to a central axis extending between the upper surface and the lower surface, and the second lower constraining surface is continuous with the second upper constraining surface and is inclined outward at an inclination angle α2 with reference to the central axis, characterised in that the first side surface comprises a first upper constraining surface and a first lower constraining surface in order as the first side surface goes from the upper surface to the lower surface, the first upper constraining surface is inclined outward at an inclination angle β1 with reference to the central axis, and the first lower constraining surface is continuous with the first upper constraining surface and is inclined inward at an inclination angle β2 with reference to the central axis. an upper surface; a lower surface; a side surface which is connected to each of the upper surface and the lower surface, and comprises a first side surface and a second side surface adjacent to each other; and a cutting edge comprising a major cutting edge located at an intersection of the upper surface and the first side surface, and a minor cutting edge located at an intersection of the upper surface and the second side surface, wherein the upper surface comprises a rake surface which is located along the major cutting edge and is inclined downward as the upper surface separates from the major cutting edge, the second side surface comprises a second upper constraining surface and a second lower constraining surface in order as the second side surface goes from the upper surface to the lower surface, wherein the second upper constraining surface is inclined inward at an inclination angle α1 with reference to a central axis extending between the upper surface and the lower surface, and the second lower constraining surface is continuous with the second upper constraining surface and is inclined outward at an inclination angle α2 with reference to the central axis, characterised in that the first side surface comprises a first upper constraining surface and a first lower constraining surface in order as the first side surface goes from the upper surface to the lower surface, the first upper constraining surface is inclined outward at an inclination angle β1 with reference to the central axis, and the first lower constraining surface is continuous with the first upper constraining surface and is inclined inward at an inclination angle β2 with reference to the central axis. a cutting edge comprising a major cutting edge located at an intersection of the upper surface and the first side surface, and a minor cutting edge located at an intersection of the upper surface and the second side surface, wherein the upper surface comprises a rake surface which is located along the major cutting edge and is inclined downward as the upper surface separates from the major cutting edge, the second side surface comprises a second upper constraining surface and a second lower constraining surface in order as the second side surface goes from the upper surface to the lower surface, wherein the second upper constraining surface is inclined inward at an inclination angle α1 with reference to a central axis extending between the upper surface and the lower surface, and the second lower constraining surface is continuous with the second upper constraining surface and is inclined outward at an inclination angle α2 with reference to the central axis, characterised in that the first side surface comprises a first upper constraining surface and a first lower constraining surface in order as the first side surface goes from the upper surface to the lower surface, the first upper constraining surface is inclined outward at an inclination angle β1 with reference to the central axis, and the first lower constraining surface is continuous with the first upper constraining surface and is inclined inward at an inclination angle β2 with reference to the central axis.