To eliminate forming of a strip mark in materials to be bookbound by a pressurizing contact of a chain even when the materials are made of carbon sheet or sheet capable of copying.

In the bookbinder comprising a sandwiching conveyor sandwiching materials to be bookbound between an upper conveyor and a lower conveyor inclined at a predetermined angle to a horizontal surface to convey it, and a back size coating roller for coating a back of the materials during conveying by the conveyor, both the conveyors of the conveyor are formed of chain conveyors, and an elastic cover member 3 is mounted at an outside of each link member (a) for forming chains 32, 39 into contact with the materials at least one side of the conveyors.