Disclosed is a multilayer body (10) having a resin layer (2) internally containing a core portion (1) which is composed of a fiber base, and a metal layer (3) joined to one side of the resin layer. The fiber base has a thickness of not more than 25 [mu]m, and the metal layer has an opening (31) corresponding to a via hole to be formed in the resin layer. Also disclosed is a method for producing asubstrate, which comprises a step for forming a via hole (23) in the resin layer by irradiating the multilayer body with a laser (9), and a step for removing the metal layer from the multilayer body after formation of the via hole. Further disclosed is a substrate (100) produced by such a method. Still further disclosed is a semiconductor device (12) which is obtained by mounting a semiconductor element (121) on the substrate.