Abstract: The known double facer of the type that when a single-faced cardboard sheet and a liner are being pinched and conveyed by a belt on the upper side and a heating box on the lower side, pressurized air is fed from an air chamber onto the back side of the belt to press the single-faced corrugated cardboard sheet and the liner onto the side of the heating box and thereby the single-faced corrugated cardboard sheet and the liner are made to stick to each other, is improved in that the side walls of the air chamber are supported in a movable manner in the widthwise direction of the sheet so that the width of the air chamber can be adjusted according to the width of the corrugated cardboard sheet to be produced to avoid the side edge portions of the corrugated cardboard sheet from crushing and insufficiently sticking. This results in reduction of unacceptable sheet products and economy of a driving power expense.