To provide a process for producing a stable modified sol to be used for a component of a hard coat agent to be applied to the surface of plastic lenses or for other applications. It is a sol containing modified metal oxide particles which have a particle size of from 4.5 to 60 nm and which comprise, as nuclei, colloidal particles (A) having a particle size of from 4 to 50 nm and comprising stannic oxide particles or composite particles of stannic oxide particles and zirconium oxide particles in a weight ratio of ZrO2:SnO2 being from 0:1 to 0.50:1, and, as applied on their surface, an alkylamine-containing Sb2O5 colloidal particles (B1), or composite colloidal particles (B2) of diantimony pentoxide and silicon dioxide, or tungsten oxide/stannic oxide/silicon dioxide composite colloid (B3), wherein the weight ratio of (B)/(A) is from 0.01 to 0.50 on the basis of the weight ratio of their metal oxides.