Abstract: The method comprises a masking step of closing by masking tapes a gap formed between a roof rail member provided at the inner periphery of an existing roof panel and the roof panel, a linear joint portion between the roof rail and pillar inner panels, and holes provided for securing parts to the roof rail. The method also includes a step of arranging a wire harness assembly at predetermined position a lining skin member below the roof panel by securing and fitting welts provided around the periphery of the lining skin member to the roof panel. A foaming mold setting step is then carried out by mounting a foaming mold having the same shape as a final shape of the lining skin member from a compartment of the vehicle. A heating step is then performed to preheat the whole ceiling portion to a predetermined temperature and a foaming composition injecting step of injecting a foaming composition into a spaced formed between the roof panel and the lining skin member is carried out.