A substrate processing method can securely form a metal film by electroless plating on an exposed surface of a base metal, such as interconnects, with increased throughput and without the formation of voids in the base metal. The substrate processing method includes: cleaning a surface of a substrate having a base metal formed in the surface with a cleaning solution comprising an aqueous solution of a carboxyl group-containing organic acid or its salt and a surfactant as an additive; bringing the surface of the substrate after the cleaning into contact with a processing solution comprising a mixture of the cleaning solution and a solution containing a catalyst metal ion, thereby applying the catalyst to the surface of the substrate; and forming a metal film by electroless plating on the catalyst-applied surface of the substrate.