The objective of the present invention is to provide: an aqueous coating composition which has excellent adhesiveness to a plastic substrate and which is capable of forming a multilayer coating film, which has excellent finishing property and water resistance and in which mixing of the layers does not occur even when preheating is not conducted after coating and the next process of applying a finishing coat is performed thereafter; and a coating method using the composition. This aqueous coating composition is characterized by comprising an aqueous dispersion of a modified polyolefin (A), an aqueous acrylic resin (B) and a blocked polyisocyanate compound having a specific blocked isocyanate (C).