An indexable rotary cutting tool having inserts detachably attached to a tool holder, (a) said insert comprising an upper surface, a lower surface opposing said upper surface and coming into contact with a seat surface of said tool holder, side surfaces connecting said upper surface to said lower surface, a center hole penetrating said insert from said upper surface to said lower surface for receiving a fastening screw, and cutting edges (11, 12, 13) formed along ridgelines of said upper surface and said side surfaces; said insert being in a polygonal shape rotationally symmetric around a center axis P of said center hole, with corner portions having angles of 90° or less; each of said cutting edges (11, 12, 13) being constituted by a corner cutting edge formed in each corner portion, and a bottom cutting edge and a peripheral cutting edge formed along ridgelines on both sides of said corner cutting edge; and (b) in a state where said inserts are attached to said tool holder, each of said bottom cutting edges and said corner cutting edges having a negative radial rake angle; each of said peripheral cutting edges having a radial rake angle changing from negative to positive, and the length of a region having a positive radial rake angle in said peripheral cutting edge being 50% or more of the entire length of said peripheral cutting edge, characterised in that, said bottom cutting edge and said peripheral cutting edge having different clearance angles and a flank of said corner cutting edge having an angle-changing surface portion having a clearance angle continuously changing in a region from the bottom cutting edge to the peripheral cutting edge; and a ridge line of said upper surface extending from an upper end of said peripheral cutting edge being provided with a receding portion determining the length of said peripheral cutting edge. (a) said insert comprising an upper surface, a lower surface opposing said upper surface and coming into contact with a seat surface of said tool holder, side surfaces connecting said upper surface to said lower surface, a center hole penetrating said insert from said upper surface to said lower surface for receiving a fastening screw, and cutting edges (11, 12, 13) formed along ridgelines of said upper surface and said side surfaces; said insert being in a polygonal shape rotationally symmetric around a center axis P of said center hole, with corner portions having angles of 90° or less; each of said cutting edges (11, 12, 13) being constituted by a corner cutting edge formed in each corner portion, and a bottom cutting edge and a peripheral cutting edge formed along ridgelines on both sides of said corner cutting edge; and (b) in a state where said inserts are attached to said tool holder, each of said bottom cutting edges and said corner cutting edges having a negative radial rake angle; each of said peripheral cutting edges having a radial rake angle changing from negative to positive, and the length of a region having a positive radial rake angle in said peripheral cutting edge being 50% or more of the entire length of said peripheral cutting edge, characterised in that, said bottom cutting edge and said peripheral cutting edge having different clearance angles and a flank of said corner cutting edge having an angle-changing surface portion having a clearance angle continuously changing in a region from the bottom cutting edge to the peripheral cutting edge; and a ridge line of said upper surface extending from an upper end of said peripheral cutting edge being provided with a receding portion determining the length of said peripheral cutting edge.