The present invention relates to metal object-coating method, which method comprises coating a primer composition (I) prepared by adding an inorganic anti-corrosive agent to at least one binder component selected from the group consisting of a binder component (A) containing an epoxy resin and ant least one of amino resin and phenol resin, a binder component (B) containing a polyester resin, epoxy resin and phenol resin, and a binder component (C) containing α-olefin•α,β-ethylenically unsaturated carboxylic acid copolymer resin onto the surface of a metal object coating substrate to form a primer coating film layer, followed by coating a topcoat coating composition consisting of a thermoplastic resin based coating composition (II) onto the primer coating film layer to form a topcoat coating film layer consisting of a thermoplastic resin coating film layer; and the primer composition used in the method.