A cap metal forming method capable of obtaining a uniform film thickness on the entire surface of a substrate is provided. A method for forming a cap metal on a processing surface of a substrate provided with two or more regions having different water-repellent properties, includes: holding the substrate horizontally by a rotatable holding mechanism installed in an inner chamber; supplying a gas between the inner chamber and an outer chamber covering the inner chamber via a gas supply hole provided in a top surface of the outer chamber; forming a pressure gradient between the inner chamber and the outer chamber; and supplying a plating solution to a preset position on the processing surface of the substrate after a pressure of the gas inside the inner chamber reaches a preset value so as to form the cap metal on at least one of the regions.