Cutting insert, holder, and blade-tip-replaceable cutting tool

A cutting insert having a multi-stage columnar shape with an insert central axis as a center, the cutting insert comprising:

• a head portion having a circular cutting edge with the insert central axis as a center;
• a shaft portion disposed on a lower side of the head portion in an insert axial direction along the insert central axis and having a smaller outer diameter dimension than the head portion;
• a step portion configured to connect the head portion and the shaft portion;
• and
• an index portion disposed over a part of the step portion and a part of the head portion and recessed inward in an insert radial direction orthogonal to the insert central axis from an outer peripheral surface of each of the step portion and the head portion,
• wherein a plurality of the index portions are arranged in an insert circumferential direction around the insert central axis, and
• each of the index portions has a planar alignment surface facing outward in the insert radial direction.