Method of preparing a catalyst used for producing methacrylic acids through gas phase catalytic oxidation of methacrolein with molecular oxygen and represented by the formula:

        P a Mo b V c Ge d X e Y f Z g O h


wherein P, Mo, V, Ge and O represent phosphorus, molybdenum, vanadium, germanium and oxygen, respectively; X represents at least one element selected from the group consisting of arsenic, antimony, bismuth, zirconium, tellurium, silver and boron; Y represents at least one element selected from the group consisting of iron, copper, zinc, chromium, magnesium, tantalum, manganese, barium, gallium, cerium and lanthanum; Z represents at least one element selected from the group consisting of potassium, rubidium, cesium and thallium; and a, b, c, d, e, f, g, and h represent the atomic ratios of the respective elements, and when b = 12, then a = 0.5-3, c = 0.01-3, d = 0.01-3, e = 0-3, f = 0-3 and g = 0.01-3. The method comprises using germanium dioxide as Ge material and dissolving at least part of germanium dioxide in an aqueous solution of a hydroxide of an element represented by Z in the above formula.