To provide an installation method for a flexible machine plate and its installation device enabling the flexible machine plate to be attached to a plate cylinder easily and surely by seting the elongation of the machine plate to 100 μm or less.

A notch at a plate tip of a machine plate 70 is made to abut against a positioning pin 14 of a plate tip clamp-mechanism 12 to be fixed thereto. A machine plate press-mechanism 17 for pressing the machine plate 70 to the outer surface of the plate cylinder after the fixation, a plate edge fixing mechanism 13 and a block copy sheet 27 having a plate-deviation preventive function being wound and fixed on the plate cylinder are provided. The plate elongation in the rotating direction of the plate cylinder at installing the machine plate is set to 100 μm or less. The irregularity of a required shape making the back side of the machine plate dented by the block copy sheet 27 being press-contacted to the machine- plate back side is provided. The projection forming the irregularity consists of plural particle- groups. The average particle-diameter of the particle-groups larger than an intermediate value between the maximum and minimum particle-diameter is larger two times the average particle-diameter of the particle-groups being smaller than the intermediate value. The total numbers of the projection consisting of average-diameter particles of the particle-groups larger than the intermediate value per unit area of a maximum section area of the face paralleled to the sheet surface is 0.1% or more of the one in the unit area.