A process for producing a transparent or opaque silica glass product including mixing a silica fine powder and a cellulose derivative and injection molding the mixture, followed by degreasing treatment and baking treatment, which is characterized in that the cellulose derivative is a cellulose derivative which causes reversible thermal gelation in an aqueous solution of at least one member selected from methyl cellulose, hydroxypropylmethyl cellulose and hydroxyethylmethyl cellulose; in producing a transparent silica glass product, the cellulose derivative is added in water heated at a gelation temperature thereof or higher, and after cooling, the formed aqueous solution is kneaded with the silica fine powder; and in producing an opaque silica glass product, the cellulose derivative is added in a silica slurry containing a silica powder and heated at a gelation temperature of the cellulose derivative or higher.