The method of manufacturing a substrate includes: forming a penetrating hole in a base layer; inserting a metal dummy part in the penetrating hole; forming an insulating portion made of synthetic resin to fill a ring-shaped gap between the penetrating hole and the dummy part; forming lower insulating layers, covering the bottom surface of the dummy part, that are made of synthetic resin on the bottom surface of the base layer to be continuous with the insulating portion; forming upper insulating layers, covering the top surface of the dummy part, that are made of synthetic resin on the top surface of the base layer to be continuous with the insulating portion; forming an exposing hole by routing in the upper insulating layers to expose the top surface of the dummy part; and forming a cavity by removing the dummy part exposed through the exposing hole by etching.