This device is equipped with a first conveyor 1 and a second conveyor 12 disposed between the first conveyor and a bookbinding device 13. The first conveyor is operated continuously and conveys book blocks B toward the bookbinding device while also being capable of accumulating the book blocks upon the conveyance surface 4. A first stopper 23 is disposed at the entry end of the second conveyor, and a second stopper 28 is disposed along the path of the book blocks upon the conveyance surface of the first conveyor. The first and second stoppers can assume a holding position for holding back the book blocks and a retracted position for allowing the book blocks to pass. The second conveyor and the first stopper operate in synchronization with a book block intake operation of the bookbinding device. The second stopper assumes the holding position when the number of book blocks accumulated on the upstream side of the first stopper exceeds a preset limit value, or when the difference in the thicknesses of book blocks conveyed after one another exceeds a preset reference value.