A process for producing methacrylic acid by the vapor phase catalytic oxidation of methacrolein with molecular oxygen or a molecular oxygen-containing gas comprises using a catalyst of multi-elemental complex oxide having the formulaMo.sub.a P.sub.b As.sub.c V.sub.d Cu.sub.e Ce.sub.f Cr.sub.g O.sub.hwherein a, b, c, d, e, f, g and h respectively represent atomic ratios and a=12, b=0.5 to 3.0, c=0.01 to 1.2, d=0.01 to 2.0, e=0.01 to 2.0, f=0.01 to 1.2, g=0 to 2.0; and h is decided on the valences of the other components such as usually 37 to 60 which is prepared with or without a reduction using a dibasic carboxylic acid, an oxycarboxylic acid, mannitol or pyrogallol.