. "To prevent damage of the cutting knife 80, it is desirable for the mat 94 to be formed of a soft material such as rubber, urethane, or mold.Next, the operation of the trimmer unit D will be described.First, when the bookbinding sheet bundle P3 is conveyed from the rotary stage 60 by the above-mentioned gripper unit 70 shown in FIG. 8C, the trimmer unit D starts the vertical motor 88 shown in FIG." . . .