Kansai Paint Co. has been awarded a patent for a water-based primer composition comprised of an aqueous dispersion formed by dispersing an unsaturated carboxylic acid- or acid anhydride-modified polyolefin having a melting point of 60°-110° C and a weight-average molecular weight within a range of 30,000-200,000, and unsaturated carboxylic acid- or acid anhydride-modified polyolefin having a melting point of 40°-90° C and a weight-average molecular weight within a range of 30,000-200,000, in an aqueous medium, the melting point of the modified polyolefin being higher than that of the modified polyolefin by at least 10° C; the composition further containing at least one resin selected from the group consisting of aqueous urethane resin, aqueous acrylic resin and aqueous polyester resin and the weight ratio of solid contents of the aqueous dispersion/resin being within a range of 20/80-80/20.