To provide flexible printing plate mounting method improving the accuracy of a printing plate mounting position and capable of preventing the shift of a printing plate at the time of printing and the scumming of a non-image part.

This flexible printing plate mounting apparatus is equipped with a plate tip positioning mechanism, which comprises a plate tip clamp mechanism for clamping the plate tip provided with three or more notches of the flexible printing plate having an uneven back surface and the positioning pins provided to the plate tip clamp mechanism so as to be opposed to the notches of the printing plate, a printing plate pressing mechanism, a plate rear fixing mechanism and a block copy sheet having an uneven shape at least on the single surface thereof and wound around a plate cylinder to be fixed thereto so that the surface having the uneven shape becomes a printing plate side. This mounting apparatus is used in the printing plate mounting method for mounting the flexible printing plate on the plate cylinder and the tip of the printing plate is brought into contact with the positioning pins to be positioned and the plate cylinder is rotated in a state releasing the plate rear of the printing plate while pressing the printing plate by the printing plate pressing mechanism to wind the printing plate around the plate cylinder and, after the winding of the printing plate is completed, the plate rear is fixed to set the elongation of the flexible printing plate in the maximum printing length in the rotary direction of the plate cylinder at the time of mounting of the printing plate to 100 μm or less.