It is intended to provide a stable sol containing zirconium oxide-tin oxide composite colloids to be used for improving the performance such as abrasion resistance, transparency, adhesiveness, water resistance or weather resistance of a dried film obtained by applying a hard coating agent containing colloidal particles to the surface of plastic lens. A production method includes the steps of obtaining a sol containing colloidal particles (A1) in which the surface of particles in a sol containing hydrothermally-processed zirconium oxide-tin oxide composite colloids (A) is coated with amine-containing Sb2O5 colloidal particles; obtaining a sol containing colloids (A2) coated with amine-containing Sb2O5 colloidal particles by performing a hydrothermal treatment at a temperature of from 200 to 350° C.; and mixing the sol containing the colloidal particles (A2) and a sol containing tungsten oxide-tin oxide-silicon dioxide composite colloidal particles (B2) and aging the resulting aqueous medium at 20 to 100° C.