The invention relates to a coating composition for preparing a back coating film of a mirror. The mirror has a glass substrate, a silver mirror film formed on a back surface of the glass substrate, a metallic protective film formed on the silver mirror film, and the back coating film formed on the metallic protective film. The coating composition comprises: a binder consisting of 40 to 85 parts by weight of the total of an epoxy resin and a hardener of the epoxy resin, and 15 to 60 parts by weight of at least one selected from the group consisting of a ketone formaldehyde resin and its derivatives; and a pigment blend containing 7-55 wt % of a corrosion-inhibitive pigment based on the weight of the pigment blend. The weight ratio of the pigment blend to the binder is in the range from 1.2 to 4.0. The back coating film is superior in chemical resistance and corrosion resistance even if a lead-free corrosion-inhibitive pigment is used.