A cutting insert (130) is formed to be rotationally symmetric with respect to an axis line (C3) of a height direction and to be planarly symmetric with respect to a virtual plane of the insert (VS1), an axis line (C2) of a traverse direction is gradually inclined toward the front of a rotational direction around which a workpiece (W) rotates moving toward a first traverse direction (C2A), an axis line (C1) of a longitudinal direction extends toward a lower surface side of an insert body (131) so as to approach a virtual plane of the tool moving toward a first longitudinal direction (C1A), and one corner portion (143C) in the other cutting edge (132B) is disposed further toward a first traverse direction (C2A) than one corner portion (143A) in one cutting edge (132A).