A method for manufacturing a composite sheet includes a first step of, in a state that a curved part of a first sheet whose one surface rises and whose another surface is recessed and that a second sheet is overlaid on the first sheet, revolving a first roller to convey the first sheet and the second sheet； a second step of passing the sheets between the first roller and a second roller revolving so that a pressing part protruding outward in the radial direction from the outer peripheral surface of the second roller is inserted into the recess of the first roller； forming, in a region within the second sheet opposing the curved part, a protruding part which is surrounded by a flat part； and a third step of joining a boundary part of the first sheet to the second sheet.
