To avoid the uselessness renewing usable molding slurry and to improve the productivity by separating the molding slurry using zirconia filler and zirconia acetate binder into zirconia and supernatant liquid, measuring the sp. gr. of the supernatant liquid and controlling the degree of degradation of the molding slurry.

It is desirable that a precision casting mold is a lost wax precision casting mold for Ti and Ti alloy. It is favorable that the molding slurry is primary layer molding slurry. It is favorable to use a centrifugal separator for the separation of the zirconia and the supernatant liquid. The acetic acid content is gradually vaporized from the supernatant liquid consisting essentially of zirconyl acetate and changed into zirconia-rich state and the sp. gr. of the supernatant liquid is gradually increased to lower the function as the binder. It is desirable to renew into the new primary layer molding slurry before the sp. gr. of the supernatant liquid exceeds 1.35.