Abstract: A printing-plate mounting-position instruction system instructs a worker where to mount a plurality of printing plates in a rotary printing press. The system includes a controller, a reader, a judging device, and a display device. The controller creates assignment information for assigning printing plates to mounting positions on each of plate cylinders used in each of printing units on the basis of production instruction information. The reader reads a mark on a printing plate for printing-surface information. The judging device is connected to the controller and the reader and collates the assignment information received from the controller and the printing-surface information received from the reader, thereby judging the location where the printing plate is to be mounted. The display device displays the result of the judgment.