PURPOSE: To obtain easily a bellows type cover comprising a thick-walled section and a thin-walled bellows section, by extruding a cylindrical body having a thin-walled section and a thick-walled section that is integrally continued from the thin-walled section through an annular nozzle whose gap can be controlled, clamping molds with the cylindrical body therein, and blowing a fluid into the cylindrical body.

CONSTITUTION: To produce the bellows type cover having the thin-walled bellows section 2' and the thick-walled cylindrical section 3', a thermoplastic resin material is extruded from the nozzle whose gap C can be controlled so that the cylindrical body 1 having the thin-walled cylindrical section 2 and the thick- walled cylindrical section 3 continuously integral with said thin-walled cylindrical section 2 is formed, then the molds D, D are closed with the cylindrical body 1 therein, and then a fluid is injected into the cylindrical body 1 so as to form the thin-walled section 2' into bellows and the thick-walled section 3' into a desirablly shaped cylindrical body.