To make crystal grain on the surface layer of a product fine by forming a shell mold using primary slurry containing crystal nucleus generating agent composed of cobalt aluminate or cobalt oxide, thereafter, keeping the shell mold to high temp. and casting molten supper alloy into a cavity, at the time of precision-casting and Ni.base supper alloy.

At the time of forming the shell mold, a wax mold is dipped into the primary slurry containing >6% the cobalt aluminate or the cobalt oxide as the crystal nucleus generating agent and 25-50% filling material. After forming the coating film, the ordinary refractory slurry is coated on the coating film without drying the coating film to make the shell mold. After dewaxing, the mold is preheated in the temp. range of 750-1250°C and the molten supper alloy is poured at the casting temp. of the liquidus temp. to the liquidus temp. +15°C and air-cooled. By this method, the surface layer crystal grain in the precision cast product can be fined and the crystal structure near the forged product, is obtd. Then, after shaking out the mold, as necessity requires, the high temp. HIP treatment can be executed.