A double-sided milling cutting insert has usable upper and lower faces. The upper face has a polygonal shape with curved edges. The lower face has a rotationally symmetrical relation to the upper face. Flank faces connect edges of the upper face to corresponding edges of the lower face. Upper cutting edges are formed on one of the edges of the upper face. The upper chip-forming portions have rake faces inclined downwardly from the upper cutting edges inwardly of the cutting insert and a plurality of protrusions arranged along inner boundaries of the rake faces for deflecting chips. When the cutting insert is mounted on the cutting tool, a width of the rake face of the upper chip-forming portion adjacent to an edge portion forming a main cutting edge gradually increases in a direction going away from a rotational axis of the cutting tool.