To provide a flexible printing plate mounting method capable of certainly and easily mounting a flexible printing plate on a plate cylinder and setting the elongation of the printing plate to 100 μm or less.

A printing plate pressing mechanism 17 for bringing the notch provided to the plate front of the printing plate 70 into contact with the positioning pin 14 of a plate front clamp mechanism 12 to fix the same, a plate rear fixing mechanism 13 and a block copy sheet 27 having a plate shift preventing function are provided. The elongation of the plate in the rotary direction of the plate cylinder at the time of mounting of the printing plate is set to 100 μm or less and the block copy sheet 27 is brought into contact with the rear surface of the printing plate under pressure to provide a projection having a required shape for depressing the rear surface of the printing plate on the surface of the printing plate and the contact area of the projection with the rear surface of the printing plate is 0.6-3.0% of the area of the rear surface of the printing plate and the projection is formed from a non-spherical filler with a mean length of 1-200 μm.