Cutting insert and cutting tool

This cutting insert (1) is a plate-shaped cutting insert (1) having a center line (O), and is provided with: an upper surface (2) that constitutes one of faces facing each other in the axial direction along the center line (O); a lower surface (3) constituting the other one of the surfaces facing each other; a side surface (4) connecting the upper surface (2) and the lower surface (3) in the axial direction; and a mounting hole (6) penetrating in the axial direction, the side surface (4) extending obliquely inward in the radial direction perpendicular to the center line (O) from the upper surface (2) toward the lower surface (3) in the axial direction, the lower surface (3) having: a seating surface (3a) having a plane perpendicular to the center line (O); and a base (3b) having, on the outer peripheral edge of the seating surface (3a), a protruding portion protruding further in the axial direction than the seating surface (3a) in a direction away from the upper surface (2), the top portion (3bc) of the protr