To suppress unfavorable rise of viscosity in aqueous slurry using frit which easily changes in the lapse of time.

At first, the frit is dry-pulverized to prescribed grain size in a dry state corresponding to usage purpose of the slurry. A silane coupling agent, a silane monomer, a silicon oil or a silicon resin are respectively individually or together added to the frit and the mixture is sufficiently blended to obtain coating-treated frit. Otherwise the silane coupling agent, the silane monomer, the silicon oil or the silicon resin are respectively individually or together added to the frit and the mixture is dry-pulverized. The coating-treated frit particles are subjected to heating treatment and then is blended with prescribed mill additive and water to prepare the slurry.