To provide a method of forming a multi-layer paint film with which, even when applying three paint layers and baking the paint layer only once, it is possible to achieve a paint film appearance the same as that with the conventional baking twice method. [Means of Resolution] A method of forming a multi-layer paint film in which an aqueous first base-paint is painted on a base material on which an electro-deposition paint has been coated and a first base-paint layer is formed, an aqueous second base-paint is painted on said first base-paint layer as a wet-on-wet system and a second base-paint layer is formed and, moreover, a clear-paint is painted on the second base-paint layer as a wet-on-wet system and a clear-paint layer is formed and then the three paint layers are all baked at the same time which is characterized in that the aqueous first base-paint includes as essential components hydroxyl group containing non-ionic resin where ethylene oxide units and/or propylene oxide units are included in an amount of from 4 to 15 mass % in the resin and of which the resin acid value is less than 15 mgKOH/g and the hydroxyl group value is from 10 to 100 mgKOH/g, and crosslinking agent.