PURPOSE: To extremely facilitate forming of a rugged pattern, by coating an adhesive to the position, located facing a recess being formed, of a skin and a cushion pad to press a parison, made of a thermoplastic resin, against the joint thereof by use of molds.

CONSTITUTION: A skin 1 and a cushion pad 2 are joined together with an adhesive 3. A coating is made such that a position where the adhesive 3 is applied is located facing the part where a recess 6 is formed in the skin 1. A joint 4 of the skin 1 and the pad 2 is inserted into molds 12 and 13 of a blow molder 11. A parsion 5' made of a thermoplastic synthetic resin is injected between the molds 12 and 13, and with the molds 12 and 13 closed, a compressed air is blown in the parison 5'. By the compressed air, the parison 5' is forcibly pressed against the side where it is brought into firm contact with the molds 12 and 13. The joint 4 is forcibly pressed by the parison 5' for expansion, but since there is a difference in expansion between the adhesive-coated part and the other part, the recesses 6 are formed in the skin 1. Simultaneously, a synthetic resin frame 5 is formed with the parison 5'.