To prevent a trouble during discharging a printing plate even when a low-strength master is used.

A master 8 is fitted on a plate cylinder 1 with one end held by a movable clamper 7 on the plate cylinder 1, and includes: a pair of plate discharge rollers including an upper side plate discharge roller 17 and a lower side plate discharge roller 17 each equipped with an elastic body facing each other, pressed and held freely rotatably, and a plate discharge box 19 for storing used masters 8 removed from the plate cylinder 1. The lower side plate discharge roller 18 located higher among the pair of plate discharge rollers is constituted to be brought into contact with the plate cylinder 1 during plate discharge operation. The pair of the plate discharge rollers 17 and 18 are driven so that after the upper side plate discharge roller 17 is brought into contact with the plate cylinder side, the clamper 7 is opened, and after the tip end of the master 8 held is opened, the pair of the plate discharge rollers 17 and 18 starts rotation.