A milling insert (20, 20A, 20B, 20C, 20D) configured to be detachably attached to a tip end portion of a tool body to be rotationally driven around a revolution axis, the milling insert comprising: an insert body having a polygonal tubular shape, the insert body comprising: rake faces provided on one polygonal surface of the insert body; flanks composed of circumferential surfaces adjacent to the polygonal surface; and cutting edges defining a ridge line where the rake faces and the flanks intersect each other, wherein the rake faces include serrations having pluralities of mount portions and valley portions that are arrayed alternately to intersect the cutting edges, when the flanks are viewed from above, the ridge line has a waveform shape in which bottom portions and top portions are formed into concave circular arc portions and convex circular arc portions, respectively, and are alternately repeated, both of the concave circular arc portions and the convex circular arc portions are a 1/4 circular arc or more to a 1/3 circular arc or less, characterized in that each of the concave circular arc portions is unevenly located at a position between two adjacent circular arc portions at a distance closer to one of the two convex circular arc portions (23a, C), and at a distance farther than the closer distance from the other of the two convex circular arc portions (23a, A), wherein the ridge line is formed with a pitch that gradually narrows at a time of being traced with respect to the concave circular arc portion toward a direction where the convex circular arc portion is positioned at the closer distance to the concave circular arc portion, and the interval of sequential pitches is set so as to become gradually narrower (R1 > R2 > R3 >...) in the direction from the inner circumference side of the tool which is closer to the revolution axis toward the outer circumference side thereof in a state where the milling insert (20, 20A, 20B, 20C, 20D) is mounted on the tool body. an insert body having a polygonal tubular shape, the insert body comprising: rake faces provided on one polygonal surface of the insert body; flanks composed of circumferential surfaces adjacent to the polygonal surface; and cutting edges defining a ridge line where the rake faces and the flanks intersect each other, rake faces provided on one polygonal surface of the insert body; flanks composed of circumferential surfaces adjacent to the polygonal surface; and cutting edges defining a ridge line where the rake faces and the flanks intersect each other, wherein the rake faces include serrations having pluralities of mount portions and valley portions that are arrayed alternately to intersect the cutting edges, when the flanks are viewed from above, the ridge line has a waveform shape in which bottom portions and top portions are formed into concave circular arc portions and convex circular arc portions, respectively, and are alternately repeated, both of the concave circular arc portions and the convex circular arc portions are a 1/4 circular arc or more to a 1/3 circular arc or less, characterized in that each of the concave circular arc portions is unevenly located at a position between two adjacent circular arc portions at a distance closer to one of the two convex circular arc portions (23a, C), and at a distance farther than the closer distance from the other of the two convex circular arc portions (23a, A), wherein the ridge line is formed with a pitch that gradually narrows at a time of being traced with respect to the concave circular arc portion toward a direction where the convex circular arc portion is positioned at the closer distance to the concave circular arc portion, and the interval of sequential pitches is set so as to become gradually narrower (R1 > R2 > R3 >...) in the direction from the inner circumference side of the tool which is closer to the revolution axis toward the outer circumference side thereof in a state where the milling insert (20, 20A, 20B, 20C, 20D) is mounted on the tool body. the ridge line is formed with a pitch that gradually narrows at a time of being traced with respect to the concave circular arc portion toward a direction where the convex circular arc portion is positioned at the closer distance to the concave circular arc portion, and the interval of sequential pitches is set so as to become gradually narrower (R1 > R2 > R3 >...) in the direction from the inner circumference side of the tool which is closer to the revolution axis toward the outer circumference side thereof in a state where the milling insert (20, 20A, 20B, 20C, 20D) is mounted on the tool body.