A novel method for preparing a heteropolyacid catalyst containing a heteropolyacid composed of molybdophosphoric acid and/or molybdovanadophosphoric acid, or a salt of the heteropolyacid, is provided. The method comprises preparing an aqueous solution or aqueous dispersion which (1) contains the nitrogen-containing heterocyclic compound, nitrate anions and ammonium ions, (2) the ammonium ion content not exceeding 1.7 mols per mol of the nitrate anion content, and (3) the ammonium ion content not exceeding 10 mols per 12 mols of the molybdenum atom content by mixing raw materials containing the catalyst-constituting elements with the nitrogen-containing heterocyclic compound in the presence of water, drying and calcining the same. This heteropolyacid catalyst excels over conventional catalysts in performance, life and strength.