A blade mount is attached or detached without bolts to save the time for the attaching and the detaching. Consequently, the operation efficiency of the rotary die cutter is enhanced, which is intended to achieve. Two fixing rings are freely fitted to a knife cylinder and the driving unit rotates the screw axis to move moving mounts. Thereby, yoke plates disposed on the moving mounts move the fixing rings to the end portion of a blade mount, and then an activated unit activates fixing units to fix the fixing rings to the outer circumference face of the knife cylinder, so that the blade mount is fixed. After the fixing rings are fixed, the yoke plates are separated from the fixing rings, so that abrasion of the yoke plates can be avoided.