A method for making a casing (1) with protective bellows (2, 2A) for transmission device, such as a transmission joint, includes at least two shafts mobile axially and/or at an angle relative to each other, the casing (1) with bellows (2, 2A), whereon at least part of the bellows (2′) are radial, having at each open end a section (3A, 3B) for being fixed to the transmission device. The method includes moulding the casing (1) with bellows (2, 2A), using a hollow mould and at least one core, having each at least a matching helical thread delimiting the moulding space and stripping the casing (1) formed by relatively unscrewing the casing (1) and the core to obtain a single-piece casing (1). The invention is applicable to a casing with bellows by arrangement of extensible or flexible coupling elements.