To provide a manufacturing method capable of forming a trumpet-shaped sheath pipe in an extremely inexpensive manner.

Cylindrical pipes formed of a resin and having the same diameter are impregnated with a liquid held to a predetermined temperature and the end part of the impregnated pipe is opposed to a trumpet-shaped fixture and this opposed trumpet-shaped fixture is inserted in the pipe and the pipe is thermally and elastically deformed into a trumpet shape. The pipe deformed into the trumpet shape is taken out of the impregnation liquid. The taken-out pipe is cooled to form a trumpet-shaped sheath pipe.