Cutting insert and indexable rotary cutting tool

A plate-shaped cutting insert that is detachably mounted in a front end portion of a tool main body that is rotated around a central axis, the cutting insert comprising:

• a rake face;
• a flank surface;
• and
• a cutting edge that is formed along an intersection ridge between the rake face and the flank surface,
• wherein the cutting edge includes
• a bottom cutting edge that is positioned at a front end in a central axis direction and forms a convex arc shape, and
• an outer peripheral cutting edge that is aligned with a radial-direction outer end of the bottom cutting edge and forms a convex arc shape of which a curvature radius is larger than that of the bottom cutting edge,
• wherein an angle formed between a tangent line to a boundary point between the bottom cutting edge and the outer peripheral cutting edge and the central axis is less than 45°,
• wherein a curvature radius R1 of the bottom cutting edge is 0.3 to 10 mm, and
• wherein a ratio (R2/R1) of a curvature radius R2 of the outer peripheral cutting edge to the curvature radius R1 of the bottom cutting edge is 3.6 to 333.