A resin-impregnated substrate having a high heat resistance at a high temperature under soldering conditions and a small linear expansion rate is provided. The resin-impregnated substrate can be obtained by a method comprising the steps of immersing a sheet comprising an aromatic liquid-crystalline polyester fiber in an aromatic liquid-crystalline polyester solution containing an aprotonic solvent and a liquid-crystalline polyester comprising 10 to 35% by mol, with respect to the total repeating units of the polyester, of at least one repeating unit selected from the group consisting of a repeating unit derived from an aromatic diamine and a repeating unit derived from an aromatic amine with a phenolic hydroxyl group; and removing the solvent.