Spherical silica-titania-based fine particles surface-treated with silane are provided. The fine particles have a titanium atom content of 0.001 to 5% by weight, a frictional electrification with iron powder of -100 to -300 .mu.C/g, a bulk density of 0.2 to 0.4 g/ml, and a particle diameter of 0.01 to 5 .mu.m. The particles are useful as a material for an external additive for an electrostatically charged image developing toner. The toner is unlikely to cause degeneration or scratching of organic photoreceptors, displays excellent dispersibility and favorable flowability and consequently does not cause adhesion to the photoreceptor.