Method for manufacturing composite sheet

A method for manufacturing a composite sheet, comprising, in this order:

• rotating a first roller with a recess retreating inward in a radial direction from an outer peripheral surface thereof, so as to convey a first sheet having a curved part with one surface raised and another surface recessed, and a boundary part located adjacent to the curved part, the curved part being accommodated in the recess of the first roller and the boundary part being arranged along the outer peripheral surface of the first roller;
• providing a second sheet over the first sheet to cover the curved part of the first sheet;
• conveying the first and second sheets along the outer peripheral surface of the first roller while rotating the first roller;
• rotating a second roller having a pressing part protruding outward in a radial direction from an outer peripheral surface thereof, in synchronization with the first roller to pass the first sheet and the second sheet between the first roller and the second roller such that the pressing part of the second roller is inserted into the recess of the first roller in a loose manner to push the second sheet into the recess so as to form, in a region within the second sheet opposing the curved part of the first sheet, a protruding part which is surrounded by a flat part and in which an inner surface of the second sheet on a first sheet side rises to approach the curved part of the first sheet and an outer surface of the second sheet on a side opposite to the first sheet is recessed;
• and
• joining the boundary part of the first sheet to the second sheet before or after or at a same time of forming the protruding part.