A cutting tool body for indexable inserts, and a round milling insert, are configured not to cause floating or rattling upon mounting the milling insert. The tool body includes a recessed portion for fitting one of a plurality of protrusions thereto, the protrusions being formed in the milling insert to protrude in the radial direction. The recessed portion has two inner surfaces having curved portions disposed so the distance between them gradually decreases at the contact area between the recessed portion and the protrusion fitted thereto. The milling insert includes protrusions protruding radially, any of which fit into a recessed portion on a mounting seat of the tool body. The protrusions each have two curved outer surfaces arranged so the distance between them gradually decreases toward a tip of the protrusion, at the contact area between the protrusion and the recessed portion into which the protrusion is fitted.