To produce methacrylic acid in high selectivity and in high yield by catalytic vapor phase oxidation of isobutane.

In this process, a catalyst including compound oxide from (i) a sparingly water-soluble hetero polyacid salt and (ii) phosphorus, molybdenum and vanadium. The component (i) has an elementary composition (except oxygen) represented by the following general formula (1) : AaBbMocWdVe (A is a cation and at least one element selected from potassium, rubidium, cesium, and thallium; B is the central element of heteropolyacid and at least one selected from silicon, phosphorus, and arsenic; Mo, W and W represents individually a ligand atom of the heteropolyacid, namely molybdenum, tungsten and vanadium where a, b, c, d and e are each the atomic ratio of the individual components, in the case of b=1, a=3-4, c=0-12, d=0-12, e=0-3 and c+d+e=12).