Cutting insert and indexable rotary cutting tool

A plate-like cutting insert comprising:

• a substantially polygonal upper face;
• a substantially polygonal lower face;
• and
• a plurality of side faces extending between the upper face and the lower face, wherein,
• the cutting insert comprises a first cutting edge formed at an upper intersection portion where the upper face and the plurality of side faces intersect with each other;
• the first cutting edge comprises at least one curved corner cutting edge formed at a corner portion of the upper face, and at least one linear cutting edge connected to the curved corner cutting edge;
• the plurality of side faces comprise at least one main side face including a plane constituting a flank of the at least one linear cutting edge, and at least one sub side face constituting a flank of the at least one corner cutting edge;
• the at least one main side face comprises a relief angle of 0°;
• the at least one sub side face comprises a relief angle larger than 0°;
• and
• the cutting insert comprises at least one second cutting edge formed at a lower intersection portion where the at least one main side face and the lower face intersect with each other.