An upper surface of a cutting insert includes a first rake face and a second seating surface. A lower surface of the cutting insert includes a second rake face and a first seating surface. The cutting edge includes a first cutting edge portion formed at a crossing edge line portion between the first rake face and a flank face and a second cutting edge portion formed at a crossing edge line portion between the second rake face and the flank face. The first cutting edge portion and the crossing edge line portion between the first seating surface and the flank face are positioned opposite each other. The second cutting edge portion and the crossing edge line portion between the second seating surface and the flank face are positioned opposite each other.