To provide a printing plate mounting method in a printing press without requiring the remodeling of the clamp of the plate cylinder of the printing press, simplifying the alignment of a flexible printing plate, and enabling printing not inferior to a PS plate.

When the flexible printing plate is applied to the surface of the plate cylinder through a substrate in the printing plate mounting method in the printing press, the pinholes same to a plurality of the pinholes provided to the substrate in a lateral direction are provided to the flexible printing plate in a lateral direction, and the pinholes of the substrate and the flexible printing plate are aligned with each other to apply the printing plate to the plate cylinder. Further, the substrate and the flexible printing plate are fixed by aligning the pinholes of the substrate and the flexible printing plate to insert fixing pins in the pinholes.