A cutting insert, 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 being adjacent to each other; and a cutting edge comprising a major cutting edge lying along an intersection of the upper surface and the first side surface, and a minor cutting edge lying along an intersection of the first side surface and the second side surface, wherein the first side surface comprises a first constraining surface located at a middle region in a thickness direction of the cutting insert, and a rake surface which is located between the first constraining surface and the major cutting edge, and is recessed with respect to the first constraining surface and the major cutting edge, wherein a distance (Wa) between the major cutting edge and the first constraining surface is increased as going toward the minor cutting edge in a side view, characterized in that the second side surface comprises a second constraining surface located at a middle region in a thickness direction of the cutting insert, and a portion (41a4) of the first constraining surface adjacent to the minor cutting edge and a portion (42a4) of the second constraining surface adjacent to the minor cutting edge are parallel to each other in a top perspective view. 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 being adjacent to each other; and a cutting edge comprising a major cutting edge lying along an intersection of the upper surface and the first side surface, and a minor cutting edge lying along an intersection of the first side surface and the second side surface, wherein the first side surface comprises a first constraining surface located at a middle region in a thickness direction of the cutting insert, and a rake surface which is located between the first constraining surface and the major cutting edge, and is recessed with respect to the first constraining surface and the major cutting edge, wherein a distance (Wa) between the major cutting edge and the first constraining surface is increased as going toward the minor cutting edge in a side view, characterized in that the second side surface comprises a second constraining surface located at a middle region in a thickness direction of the cutting insert, and a portion (41a4) of the first constraining surface adjacent to the minor cutting edge and a portion (42a4) of the second constraining surface adjacent to the minor cutting edge are parallel to each other in a top perspective view. a first constraining surface located at a middle region in a thickness direction of the cutting insert, and a rake surface which is located between the first constraining surface and the major cutting edge, and is recessed with respect to the first constraining surface and the major cutting edge, wherein a distance (Wa) between the major cutting edge and the first constraining surface is increased as going toward the minor cutting edge in a side view, characterized in that the second side surface comprises a second constraining surface located at a middle region in a thickness direction of the cutting insert, and a portion (41a4) of the first constraining surface adjacent to the minor cutting edge and a portion (42a4) of the second constraining surface adjacent to the minor cutting edge are parallel to each other in a top perspective view.