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. <EOS>