The present invention provides a method for forming a coating film on a plastic substrate which comprises: (1) coating the plastic substrate with an aqueous white primer (A) having a water content of 15 to 48 wt. % to form a coating film having an L* value of 80 or more, (2) heating the coating film of the primer (A) to adjust the water content of the film to 1 to 10 wt. % and the surface electrical resistivity value of the film to less than 10.sup.9 .OMEGA./square, (3) electrostatically coating the coating film of the primer (A) with a thermosetting clear colored coating composition (B), (4) electrostatically coating the uncured coating film of the coating composition (B) with a thermosetting clear coating composition (C), and (5) baking the three-layer coating film to obtain a cured multilayer coating film having an N value of 8.5 or more.