PURPOSE: To enable to obtain excellent molding properties and electromagnetic- shielding effect, by closing a mold and mutually welding thermoplastic layers, wherein parisons are facing each other and electromagnetic-shielding properties are given.

CONSTITUTION: A split mold 1, 2 consists of a male mold 1 that has a cavity portion 11 and a female mold 2 that has a cavity portion 12. EP and TP are melted and kneaded respectively by their extruding machine, joined with each other within an extruding die and a multi-layer parison 3 in a plastic state, whose inside layer 3a and outside layer 3b are composed of EP and TP respectively, is extruded. The molds 1, 2 are closed to an extent a little before completion of the closure and the cavity portion 11 is made to enter into the cavity portion 12. The parison 3 is made to expand up to shapes corresponding to the cavity portion 11 and the cavity portion 12 by introducing a pressure fluid such as compressed air into the parison 3 surrounded by the molds 1, 2 through a blowing nozzle 4 and a form of an external appearance of a housing component is molded. Then welding is performed so that the EP layers confronted to each other are compensated with each other by closing the molds 1, 2 further.