Abstract: Amorphous silica-titania composite oxide powder is powder untreated with a surface treatment agent and consisting of amorphous silica-titania composite oxide particles, wherein: a refractive index at a measurement wavelength of 589 nm is not less than 1.46; a volume-based cumulative 50% diameter is 0.1 ?m to 2.0 ?m; and a content of particles having a particle size of not less than 5.0 ?m is not more than 10 ppm, and wherein, in a case where the powder is dried in an atmospheric air at 110° C. for 12 hours, and powder thus dried is stored for 24 hours at a temperature of 25° C. and a relative humidity of 85% so as to absorb moisture, a water absorption rate is not more than 0.8% by mass as calculated from a mass X before moisture absorption and a mass Y after the moisture absorption in accordance with the formula: (Y?X)/X×100.