A cutting insert comprising: a first end surface and a second end surface which are opposed to each other; a peripheral side surface which extends so as to connect the first end surface and the second end surface; a first cutting edge which is arranged in an intersecting edge between the first end surface and the peripheral side surface; and a second cutting edge which is arranged in an intersecting edge between the second end surface and the peripheral side surface, wherein: the peripheral side surface includes a first flank (4a1) which is connected to the first cutting edge, an intermediate peripheral side surface portion which is arranged closer to the second end surface than the first flank (4a1) and a second flank (4a2) which is connected to the second cutting edge; the first flank (4a1) extends in a direction perpendicular to the first end surface; the second flank (4a2) extends in a direction perpendicular to the second end surface; the intermediate peripheral side surface portion protrudes outward from the first flank (4a1) and the second flank (4a2); and the intermediate peripheral side surface portion is located further outward than the first flank (4a1), throughout the peripheral side surface, when viewed from the direction facing the first end surface, and than the second flank (4a2), throughout the peripheral side surface, when viewed from the direction facing the second end surface. a first end surface and a second end surface which are opposed to each other; a peripheral side surface which extends so as to connect the first end surface and the second end surface; a first cutting edge which is arranged in an intersecting edge between the first end surface and the peripheral side surface; and a second cutting edge which is arranged in an intersecting edge between the second end surface and the peripheral side surface, wherein: the peripheral side surface includes a first flank (4a1) which is connected to the first cutting edge, an intermediate peripheral side surface portion which is arranged closer to the second end surface than the first flank (4a1) and a second flank (4a2) which is connected to the second cutting edge; the first flank (4a1) extends in a direction perpendicular to the first end surface; the second flank (4a2) extends in a direction perpendicular to the second end surface; the intermediate peripheral side surface portion protrudes outward from the first flank (4a1) and the second flank (4a2); and the intermediate peripheral side surface portion is located further outward than the first flank (4a1), throughout the peripheral side surface, when viewed from the direction facing the first end surface, and than the second flank (4a2), throughout the peripheral side surface, when viewed from the direction facing the second end surface. the peripheral side surface includes a first flank (4a1) which is connected to the first cutting edge, an intermediate peripheral side surface portion which is arranged closer to the second end surface than the first flank (4a1) and a second flank (4a2) which is connected to the second cutting edge; the first flank (4a1) extends in a direction perpendicular to the first end surface; the second flank (4a2) extends in a direction perpendicular to the second end surface; the intermediate peripheral side surface portion protrudes outward from the first flank (4a1) and the second flank (4a2); and the intermediate peripheral side surface portion is located further outward than the first flank (4a1), throughout the peripheral side surface, when viewed from the direction facing the first end surface, and than the second flank (4a2), throughout the peripheral side surface, when viewed from the direction facing the second end surface.