A stopper for assuredly preventing shifting of a cylindrical printing plate mounted on a plate cylinder is provided. A plate cylinder 1 has a plate-mounting surface 12 on which a cylindrical printing plate 3 is fitted from the front end side and mounted. A front stopper 4 is provided on the end portion of the front end side. The front stopper 4 is configured so as to be able to switch between a protruding orientation in which at least a portion of the stopper is positioned so as to protrude to the outside of the plate-mounting surface 12 in the radial direction and a retracted orientation in which the entire stopper is positioned to the inside of the plate-mounting surface 12 in the radial direction.