An internal molding having a predetermined shape of a hollow portion of a manifold is integrally molded by blow molding or the like, and external molding elements having a predetermined shape of the exterior of the manifold and being split into a plurality of parts are molded. The internal molding is integrated with the external molding elements by adhesion or fusion. Further, an external molding is molded, and an internal molding having a shape substantially identical with or smaller than a predetermined shape of a hollow portion of the hollow resin molding is integrally molded. Then, the internal molding is softened by arranging the internal molding inside the external molding and heating the internal molding, and the inside of the internal molding is thereafter pressurized to cause the internal molding to contact the external molding, so that the internal molding is integrated with the external molding. Still further, an internal molding is integrated with an external molding elements by an appropriate joining means. Then, a flange member is fitted to a first connecting portion formed by the respective external molding elements and the internal molding. Still further, an internal molding is integrated with an external molding elements by an appropriate joining means. Then, a seal member is inserted into a gap portion at an end portion of the hollow resin molding as well as between the external molding and the internal molding.