The object of the present invention is to provide a catalyst for producing methacrylic acid in high yield and highly selectively by subjecting methacrolein, isobutylaldehyde or isobutyric acid to gas phase catalytic oxidation, and the preparation method thereof. The catalyst contains Mo, V, P, Cu, Cs and NH4 as the essential, active components, and the feature is to use for preparing the catalyst a cesium weak acid salt or cesium hydroxide as the Cs raw material and ammonium acetate as the NH4 raw material. A coated catalyst of the present invention is obtainable by supporting the active component on an inert carrier of alumina or the like.