PURPOSE: To avoid efficiency loss and to make manufacture simple and low in cost by welding a blade to the surface of an inner and/or outer casing shell facing the blade side.

CONSTITUTION: A blade 2 is welded to the surface of an inner and/or outer casing shell 1 facing the blade side. In order to weld this blade 2 to a singular or a plurality of casing shells 1, known capacitor discharge welding or pulse welding is used, so that the blade 2 can be perfectly welded to the casing shell 1 with extremely short welding pulse without actually causing temperature rise and distortion. Since the blade 2 is welded to the inner or outer surface facing the blade 2, if welding energy is supplied from the outside from the opposite side to the casing shell 1, through the casing shell 1, a welding process itself can be carried out very easily.