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 (2, 3), 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.