Method for forming a multilayer coating film

An object of the present invention is to provide a method of producing a multilayer coating film having excellent smoothness and high distinctness of image. The method of forming a multilayer coating film according to the present invention includes the steps of: successively applying an aqueous intermediate coating composition, an aqueous base coating composition, and a clear coating composition to a substrate; and simultaneously heat-curing the resulting intermediate, base, and clear coating layers, the aqueous intermediate coating composition (X) comprising: a base resin (A); a curing agent (B); and a diester compound (C), the curing agent (B) being a polyisocyanate compound (B1) and/or a polycarbodiimide compound (B2); and the diester compound (C) being represented by formula (1): (wherein R1and R2are each independently a hydrocarbon group having 4 to 18 carbon atoms, R3is an alkylene group having 2 to 4 carbon atoms, m an integer of 3 to 25, and in oxyalkylene units (R3-O) may be the same or different).