A cutting insert comprising a first end surface , a second end surface arranged so as to be opposed to the first end surface , and a peripheral side surface extending between the first and second end surfaces , wherein: the first end surface and the second end surface each have, in a plan view, a shape comprised of one substantially quadrangular portion and two semicircular portions which are respectively joined to two opposing sides of the substantially quadrangular portion ; an intersecting edge between each of the first end surface and the second end surface and the peripheral side surface is provided with a cutting edge ; the cutting edge is formed in at least part of the semicircular portion ; a mounting hole is formed so as to penetrate substantially respective centers of the first end surface and the second end surface ; and at least part of a portion of the peripheral side surface which is adjacent to the substantially quadrangular portion of each of the first end surface and the second end surface , and at least part of a portion of the peripheral side surface which is adjacent to the semicircular portion , each have a function of coming into contact with an insert seat, wherein: the cutting edge formed on the first end surface side has a portion projected from a seating part of the first end surface in a direction extending from the second end surface to the first end surface ; and the cutting edge formed on the second end surface side has a portion projected from a seating part of the second end surface in a direction extending from the first end surface to the second end surface . the first end surface and the second end surface each have, in a plan view, a shape comprised of one substantially quadrangular portion and two semicircular portions which are respectively joined to two opposing sides of the substantially quadrangular portion ; an intersecting edge between each of the first end surface and the second end surface and the peripheral side surface is provided with a cutting edge ; the cutting edge is formed in at least part of the semicircular portion ; a mounting hole is formed so as to penetrate substantially respective centers of the first end surface and the second end surface ; and at least part of a portion of the peripheral side surface which is adjacent to the substantially quadrangular portion of each of the first end surface and the second end surface , and at least part of a portion of the peripheral side surface which is adjacent to the semicircular portion , each have a function of coming into contact with an insert seat, wherein: the cutting edge formed on the first end surface side has a portion projected from a seating part of the first end surface in a direction extending from the second end surface to the first end surface ; and the cutting edge formed on the second end surface side has a portion projected from a seating part of the second end surface in a direction extending from the first end surface to the second end surface . the cutting edge formed on the first end surface side has a portion projected from a seating part of the first end surface in a direction extending from the second end surface to the first end surface ; and the cutting edge formed on the second end surface side has a portion projected from a seating part of the second end surface in a direction extending from the first end surface to the second end surface . <EOS>