In order to produce a dental cast product having the flat surface, by blending a dispersant with a phosphate-based investing material to improve the dispersability of an aggregate powder and a binder powder so as to prevent a agglomeration of the aggregate powder and the binder powder thereby forming a wall surface of an cavity in a mold to be flat, and also to make more various kinds of powders available for the aggregate powder without restriction to quartz, cristobalite and tridymite, the phosphate-based dental investing material comprising a mixed powder including an aggregate powder and a phosphate-based binder powder, and a liquid, with an alkaline metal salt of phosphoric acid being added to at least one of the mixed powder and the liquid.