Method for producing silica-zirconia composite particles each coated with silica layer

A method for producing silica-zirconia composite oxide particles each coated with a silica layer, used as a filler for dental composite resin, which method comprising the steps of:

• reacting an alkoxide of silicon and/or a condensable compound derived from the alkoxide with an alkoxide of zirconium and/or a condensable compound derived from the alkoxide in a water-containing solvent that contains acetonitrile so that the reaction mixture contains at least 10% by mass of acetonitrile, to produce a liquid dispersion of silica-zirconia composite oxide particles dispersed in the reaction mixture at 5 to 30% by mass, and then reacting the silica-zirconia composite oxide particles dispersed in the liquid dispersion containing at least 10% by mass of acetonitrile with an alkoxide of silicon and/or a condensable compound derived from the alkoxide in the liquid dispersion of silica-zirconia composite oxide particles, to coat the surface of each silica-zirconia composite oxide particle with a silica layer.