PURPOSE: To obtain a high quality work without cracking on a shell layer and the work by making all pipes flexible, at the time of casting a cast product including the pipes as cast-in by a lost wax method.

CONSTITUTION: At the time of casting the work W contg. the pipe as cast-in, all pipes are made flexible pipes P. At the time of manufacturing a shell layer (b) by the lost wax method, as the flexible pipe P is freely expanded/shrunk, the crack is not developed on the shell layer (b). Further, even if molten metal is poured after dewaxing, the flexible pipe P is freely expanded/shrunk and the crack is not developed on the work W. In the step (c), after dipping a wax pattern (x) into slurry (b) for forming the shell layer (b), stuccoing material (silica sand)is spread and applied and dried. This process is repeated service times to form the shell layer (b) having a prescribed thickness on the surface of the wax pattern (x). By this method, the high quality work can be obtd.