Abstract: This invention provides a water-based primer composition comprising an aqueous dispersion of modified polyolefin, aqueous urethane resin and/or aqueous acrylic resin, specific diester compound and electrically conducting pigment, the content of the diester compound being specific. The water-based primer composition excels in adhesive property to plastic substrates, and use of the water-based primer composition enables formation of multilayered coating film excelling in finished appearance and water resistance, being free of layer mixing even when the top coating of the next step is carried out after the primer's application without an intervening preheating.