A three side trimmer has a trimming unit 1, an accumulating unit 3, a discharge unit 2 and a conveying mechanism 4 conveying trimmed booklets BK to the accumulating unit. The accumulating unit has a shelf 5 movable in a vertical direction, and a drive roller conveyor 7 on the shelf. The accumulating unit, in a mode, delivers the trimmed booklets to the discharge unit at every accumulation of a predetermined number of the trimmed booklets and, in another mode, accumulates the trimmed booklets without delivering.