An insert includes four identical cutting edges disposed at the intersection of top and side surfaces surface. Each cutting edge includes a first part edge disposed tangent to a first reference circle aligned about the central mounting aperture; a second part edge extending from the first part edge, the second part edge being connected to the first part edge via a convex curved portion; a third part edge disposed tangent to a second reference circle aligned about the central mounting aperture, the second reference circle having a smaller diameter than the first reference circle, the third part edge being connected to the second part edge via a concave curved portion; and a convexly curved corner portion extending from the third part edge of one of the identical cutting edges to the first part edge of another one of the identical cutting edges.

Provided is a cutting insert or the like that is capable of copy machining in a narrow space and is also highly economical. A cutting insert (3) has a first end portion (4) and a second end portion (5) formed in substantially the same shape. A cutting edge (10) of the first end portion (4) includes a curved corner cutting edge (13) and a pair of lateral cutting edges (11 and 12) disposed such that both ends (13L and 13R) of the corner cutting edge (13) are sandwiched therebetween. The pair of lateral cutting edges (11 and 12) are formed linearly and form an acute angle (Y). A side surface (9) has a corner flank (94) continuous with the corner cutting edge (13) and a flat surface (95) disposed further toward a bottom surface (8) than the corner flank (94) at a portion (93) abutting on the corner cutting edge (13).

A cutting tool includes: a cutting edge configured on an intersecting line of a rake surface and a flank; a breaker wall formed inside the cutting edge and protruding above the cutting tool; a boss part including a boss surface formed above the breaker wall; and a plurality of recessed parts formed in the boss part so as to extend toward the cutting edge. The width of the breaker wall in a direction in which the cutting edge extends along an intersecting line (including a virtual intersecting line or a virtual extension of the intersecting line) of the boss surface and the breaker wall, is greater than the width of a groove formed by the recessed part along the same intersecting line.