A milling tool includes a cutting tool insert, roughly plate-shaped and having a rake face on an upper face and a flank on a side face and a main cutting edge at an intersection ridge line part at the intersection of the rake face and the flank, fitted in the periphery of a substantially cylindrical holder removably and so that an axial rake angle of the cutting edge ridge line of the main cutting edge is positive. When the cutting tool insert is rotated about the axis of rotation of the holder, the shape of the locus of rotation of the main cutting edge in a section parallel to the axis of rotation is made an approximate circular arc shape swelling outward with progress from the ends to the center.