A detachable cutting insert for an indexable rotary cutting tool, has a cutting edge portion that includes a major cutting edge continuous with a first end of a first corner edge near a short-side-direction side surface, which is formed into a straight line. The cutting insert has a first minor cutting edge that is continuous with a second end of the major cutting edge, which is formed into a circular arc. The cutting insert has a second minor cutting edge that is continuous with a third end of the first minor cutting edge, which is formed into a straight line. In a cross-sectional view of a rake face of the second minor cutting edge, perpendicular to the second minor cutting edge, a cross-sectional ridgeline of the rake face of the second minor cutting edge is formed into a convex shape protruding above a cross-sectional ridgeline of a reference plane.