The aminosilane-modified colloidal silica dispersion contains colloidal silica particles having surfaces to which there are bound a first silyl group represented by the following formula (1): R1aSi(OR2)3-aO— and a second silyl group represented by the following formula (2): R3bSi(OR4)3-bO— and, as a dispersion medium, a mixed solvent formed of a polar solvent S1 having a dielectric constant at 20° C. of 15 or higher and lower than 60 and a non-polar solvent S2 having a dielectric constant at 20° C. of 1 or higher and lower than 15, at a mass ratio (S1/S2) of 0.3 to 6.