A cutting insert comprising: a cylindrical body portion having a first main surface , a second main surface , and an outer circumferential surface , the second main surface being opposite to the first main surface, the outer circumferential surface being continuous to each of the first main surface and the second main surface, the cylindrical body portion being provided with an insertion hole extending between the first main surface and the second main surface; a cutting portion protruding from the outer circumferential surface in a radial direction; and only three fitting portions, namely a first fitting portion , a second fitting portion , and a third fitting portion each protruding from the second main surface in an axial direction and each extending in the radial direction, wherein the cutting portion includes a rake face and a flank face continuous to the rake face, and a ridgeline between the rake face and the flank face constitutes a cutting edge , when viewed in the axial direction, the cutting portion is provided opposite to the third fitting portion relative to the insertion hole, when viewed in the axial direction, a straight line extending through an outer circumferential end of the cutting edge and a center of the insertion hole overlaps with the third fitting portion and is located between the first fitting portion and the second fitting portion, and when viewed in the axial direction, assuming that a central angle between the second fitting portion and the third fitting portion represents a first central angle (θ1), a central angle between the third fitting portion and the first fitting portion represents a second central angle (θ2), and a central angle between the first fitting portion and the second fitting portion represents a third central angle (θ3), the third central angle is smaller than the first central angle and is smaller than the second central angle. a cylindrical body portion having a first main surface , a second main surface , and an outer circumferential surface , the second main surface being opposite to the first main surface, the outer circumferential surface being continuous to each of the first main surface and the second main surface, the cylindrical body portion being provided with an insertion hole extending between the first main surface and the second main surface; a cutting portion protruding from the outer circumferential surface in a radial direction; and only three fitting portions, namely a first fitting portion , a second fitting portion , and a third fitting portion each protruding from the second main surface in an axial direction and each extending in the radial direction, wherein the cutting portion includes a rake face and a flank face continuous to the rake face, and a ridgeline between the rake face and the flank face constitutes a cutting edge , when viewed in the axial direction, the cutting portion is provided opposite to the third fitting portion relative to the insertion hole, when viewed in the axial direction, a straight line extending through an outer circumferential end of the cutting edge and a center of the insertion hole overlaps with the third fitting portion and is located between the first fitting portion and the second fitting portion, and when viewed in the axial direction, assuming that a central angle between the second fitting portion and the third fitting portion represents a first central angle (θ1), a central angle between the third fitting portion and the first fitting portion represents a second central angle (θ2), and a central angle between the first fitting portion and the second fitting portion represents a third central angle (θ3), the third central angle is smaller than the first central angle and is smaller than the second central angle. <EOS>