To provide a method for mounting a machine plate by which a flexible machine plate can be surely and easily mounted on a plate cylinder and elongation of the machine plate can be made at most 100 μm and an apparatus for mounting it.

When the plate front of the flexible machine plate 70 is fixed on a plate front clamping mechanism 12, the cutout of the plate front of the machine plate 70 is brought into contact with a positioning pin 14 of the plate front clamping mechanism 12 to position and fix it and when the machine plate 70 is wound on the plate cylinder 11 after it is fixed, a machine plate pressing mechanism 17 for pressing the machine plate 70 on the outer peripheral face of the plate cylinder by a pressing means, a plate end fixing mechanism 13 for fixing the plate end of the machine plate 70 on the plate cylinder 11 and a block copy sheet 27 with a plate deviation preventing function wound and fixed on the plate cylinder are provided and the elongation of the plate in the plate cylinder rotating direction when the machine plate is mounted is made to be at most 100 μm and the block copy sheet 27 has a durometer hardness of the surface of the block copy sheet brought into contact with the back surface of the plate of at most 65 and projections with a height of at least 1 μm are provided on the surface in such a way that the total per the maximum sectional area of the face parallel to the sheet face of the projections is at least 0.5% of the unit area and the heights of the projections are at most 50 μm.