A panel body of an instrument panel in an automobile is formed of a synthetic resin foam. A duct element made of a synthetic resin foam having a compatibility with the panel body is welded to a back of the panel body to define air-conditioning air passages between the panel body and the duct element. Thus, the instrument panel also serves as a single duct element. Therefore, it is possible to provide an inexpensive air conditioning duct device in an automobile, which can be produced from a decreased number of parts and at a decreased number of assembling steps.