PURPOSE: To impart excellent gas barrier properties, water-impermeability and surface hardness, by coating the outer surface of the wall of a biaxially stretched body of a container made of a thermoplastic polyester with a specified coating agent.

CONSTITUTION: A flame treatment, corona discharge treatment or plasma treatment is applied, if necessary, to the outer surface of the wall of a biaxially stretched body of a container made of a thermoplastic polyester. The treated surface is coated, if necessary, with a coating agent contg., as a film-forming component, a thermoplastic resin selected from ethylene/vinyl acetate copolymer, PVC, polyvinyl acetate, vinyl chloride/vinyl acetate copolymer and isocyanate polymer, and dried to form a primer layer. Then the surface is coated with a coating agent composed of a soln. contg. 5W20wt% vinylidene chloride (co) polymer as a film-forming component, and dried to form a coated synthetic resin layer of 0.5W20μm in thickness.