A pair of sub-flank surfaces is formed at a vertex angle portion of an insert body having a substantially polygonal, plate-like shape, and between the sub flank surfaces, a vertex flank surface at a vertical angle portion is formed substantially in a protrusive, curved shape, and is smoothly connected to the sub-flank surfaces in a tangential direction. The vertex flank surface at the vertex angle portion is extended across the entire thickness of the insert body, and boundary lines, between the vertex flank surface at the vertex angle portion and a pair of minor cutting edges, are parallel to each other. A second curved ridge line, which is formed between (i) the flank surface and (ii) the rake surface, and a first curved ridge line, which is formed between (i) the flank surface and (ii) the seat surface, have approximately same shape.