A cutting insert comprising: a polygonal plate-shaped insert main body that includes: two polygonal surfaces which have a polygonal shape and of which one serves as a rake face and the other serves as a seating surface; a side surface which is arranged around the two polygonal surfaces and in which a flank face intersecting with the rake face of the polygonal surfaces is formed; and a cutting edge which is formed on an intersecting ridgeline between the rake face and the flank face, wherein the insert main body has a mounting hole that penetrates the insert main body centered on an insert center line passing through centers of the two polygonal surfaces, has a rotationally symmetrical shape with respect to the insert center line, and has a front-back inversion symmetrical shape with respect to the two polygonal surfaces, wherein the cutting edge includes at least a corner edge located at a corner of the polygonal surface and a major cutting edge extending from a first end of the corner edge, wherein, in an opening of the mounting hole in the two polygonal surfaces, a plurality of protrusions protruding with respect to a boss surface formed around the opening of the mounting hole are formed at intervals in a peripheral direction in an inner periphery side region of the major cutting edge, and wherein, in the protrusions, when seen in a direction facing the polygonal surface, furthest protruding portions which protrude furthest from the boss surface are located between a straight line connecting the first end of the corner edge and the insert center line to each other and a straight line connecting a first end of the major cutting edge on a side opposite to the first end of the corner edge and the insert center line to each other, characterized in that the insert main body has a triangular plate shape in which each of the two polygonal surfaces has three corners, and wherein, when seen in the direction facing the polygonal surface, the furthest protruding portions are located in an angle range of 15° to 40° centered on the insert center line from the straight line connecting the first end of the major cutting edge on the side opposite to the first end of the corner edge and the insert center line to each other toward the corner edge side. a polygonal plate-shaped insert main body that includes: two polygonal surfaces which have a polygonal shape and of which one serves as a rake face and the other serves as a seating surface; a side surface which is arranged around the two polygonal surfaces and in which a flank face intersecting with the rake face of the polygonal surfaces is formed; and a cutting edge which is formed on an intersecting ridgeline between the rake face and the flank face, wherein the insert main body has a mounting hole that penetrates the insert main body centered on an insert center line passing through centers of the two polygonal surfaces, has a rotationally symmetrical shape with respect to the insert center line, and has a front-back inversion symmetrical shape with respect to the two polygonal surfaces, wherein the cutting edge includes at least a corner edge located at a corner of the polygonal surface and a major cutting edge extending from a first end of the corner edge, wherein, in an opening of the mounting hole in the two polygonal surfaces, a plurality of protrusions protruding with respect to a boss surface formed around the opening of the mounting hole are formed at intervals in a peripheral direction in an inner periphery side region of the major cutting edge, and wherein, in the protrusions, when seen in a direction facing the polygonal surface, furthest protruding portions which protrude furthest from the boss surface are located between a straight line connecting the first end of the corner edge and the insert center line to each other and a straight line connecting a first end of the major cutting edge on a side opposite to the first end of the corner edge and the insert center line to each other, characterized in that the insert main body has a triangular plate shape in which each of the two polygonal surfaces has three corners, and wherein, when seen in the direction facing the polygonal surface, the furthest protruding portions are located in an angle range of 15° to 40° centered on the insert center line from the straight line connecting the first end of the major cutting edge on the side opposite to the first end of the corner edge and the insert center line to each other toward the corner edge side.