Provided is a pair of plates for a rotary processing machine having the following configuration and a method of mounting the same by which the pair of plates can be mounted on the respective outer circumferential surfaces of a pair of cylinders with high precision. When a female recessed plate and a male raised plate are overlapped with each other so as to make processing portions of the female plate correspond to those of the male plate, fitting protrusions are fitted into respective fitting portions. Accordingly, the movement of the fitting protrusions is restricted both in the width direction of the plates and in the mount direction of the plates. Concurrently, guiding protrusions are respectively inserted between corresponding pairs of guide protrusions in the guiding portions. Accordingly, the guiding protrusions are guided so that relative movement of the guiding protrusions is allowed only in the mount direction.