Abstract: This cutting insert is a plate-shaped cutting insert having a center line, and includes an upper surface that forms one surface of two surfaces facing each other in an axial direction along the center line; a lower surface that forms the other surface of the two surfaces facing each other; a side surface that connects the upper surface and the lower surface in the axial direction; and a mounting hole that passes through the cutting insert in the axial direction, in which the side surface extends obliquely inward in a radial direction perpendicular to the center line from the upper surface toward the lower surface in the axial direction.