A rotary slitter includes a disc-shaped rotary slitter body having a rotation axis, opposing first and second surfaces, a circumferential surface connecting the first and second side surfaces, and a cutting portion. The cutting portion includes an insert seating construction, a groove formed in the first surface and surrounding the insert seating construction, in a view of the first surface along the rotation axis, and a positioning hole. The rotary slitter further includes a positioning screw mounted to the positioning hole and configured to adjust a position of a cutting edge of a cutting insert mounted to the insert seating construction.