Cutting insert and cutting-edge-replaceable cutting tool

A cutting insert detachably attached to an insert attachment seat formed at a tip of a tool body in a cutting-edge-replaceable cutting tool rotating around an axis, comprising:

• a rake face which faces a rotation direction of the tool body;
• a seating face which is seated on a bottom face of the insert attachment seat to face the side opposite to the rake face;
• and
• a flank face which extends around the rake face and the seating face,
• wherein two cutting edges each including an arc-shaped cutting edge part extending in an arc shape in a plan view when viewed from a direction facing the rake face and a linear cutting edge part extending to be in contact with the arc-shaped cutting edge part are formed at an intersection ridge line part between the rake face and the flank face so that the arc-shaped cutting edge part and the linear cutting edge part are located alternately in a circumferential direction of the rake face, and
• wherein each of the cutting edges includes a convex curved part moving away from the seating face and then moving close to the seating face as at least the arc-shaped cutting edge part moves away from the linear cutting edge part, and
• in a side view when viewed from a direction facing the flank face, the arc-shaped cutting edge part includes a curvature change point between a cutting edge tip of the arc-shaped cutting edge part and the most protruding point when the most protruding point indicates a point farthest from the seating face on the convex curved part.