a process for preparing a multi - component catalyst for producing methacrylic acid containing phosphorus , molybdenum and vanadium comprising using in preparing the catalyst , at least one number selected from the group consisting of the oxide , carbonates , acetates and hydroxides of each catalyst - constituting element as a source of the element ; dissolving or dispersing the selected sources of the catalyst - constituting elements in water ; adding at least one member selected from the group consisting of ammonium carbonate , ammonium hydrogen - carbonate , ammonium sulfate and ammonium hydrogen - sulfate to the resulting aqueous solution or dispersion ; removing water therefrom and then heat - treating the residual product .