PURPOSE: To make filling of sand, etc., in the inner part and taking-out of the sand after casting unnecessary by growing dendritic structure from the cast product surface and tubular worked product surface to the inside, in the cast product having cooling hole produced by insert-casting the tubular worked product in molten metal.

CONSTITUTION: This cast product is the one having the cooling hole made by executing the insert casting with the molten metal outside the tubular worked product A formed so as to pass the fluid through the inner part. Both of this tubular worked product A and the molten metal are iron-base alloy and after casting, the both are melted at a part of the interface or the whole surface. Further, the dendritic structure is extended from the cast product surface to the inside of the worked surface after finish-working just under the surface of the cast product and the cooling hole with the dendritic structure extended from the surface of the tubular worked product is obtd. By this method, the product having excellent quality and sufficiently melted with the molten metal can be manufactured and further, this cast product is the excellent one having close casting structure at the circum ferential part of the pipe and finished surface or the just under part thereof and sound quality.