Cutting insert and indexable edge rotary cutting tool

A positive-type cutting insert mountable on a tool main body rotating around a rotation axis, and having a polygonal plate shape which is rotationally symmetric with respect to a center line extending in a thickness direction, the cutting insert comprising:

• a rake face configuring one of a pair of polygonal surfaces;
• a seating surface configuring the other one of the pair of polygonal surfaces;
• and
• a side surface connecting the rake face and the seating surface to each other,
• wherein an intersection ridgeline between the rake face and the side surface has a cutting edge portion including a main cutting edge located in a side portion of the rake face, a subsidiary cutting edge connected to the main cutting edge, and a corner cutting edge connected to the subsidiary cutting edge and located in a corner portion of the rake face,
• wherein the main cutting edge extends linearly in a plan view and a side view of the cutting insert,
• the subsidiary cutting edge is located between the main cutting edge and the corner cutting edge, and is continuously connected to the main cutting edge and the corner cutting edge,
• wherein the side surface includes a flank face and a connection surface which are adjacent to each other in the thickness direction via a boundary line,
• wherein the flank face is located closer to a rake face side than the boundary line,
• wherein the connection surface is located closer to a seating surface side than the boundary line, and is located outside an extension line of the flank face,
• the flank face is an inclined surface having an acute angle of inclination with respect to the rake surface,
• the seating surface and a portion of the flank face which is connected to the main cutting edge function as restriction portions in contact with the tool main body, and
• the connection surface increases an area of the seating surface by being located outside an extension line of the flank face.