PURPOSE: To obtain advantageously the titled compound, by using a specific catalyst consisting essentially of phosphorus, molybdenum and an alkali metal, etc. and oxygen in catalytically oxidizing isobutyl alcohol in the vapor phase with molecular oxygen.

CONSTITUTION: Isobutyl alcohol is catalytically oxidized in the vapor phase with molecular oxygen in the presence of a catalyst expressed by the formula (X is alkali metal or/and Tl; Z is Zn, Al, Sb, S, In, U, Cd, Ca, Ag, Cr, Si, Ge, Co, Zr, Sn, Sr, Se, W, Ta, Ti, Fe, Te, Cu, Th, Pb, Nb, Ni, V, Ba, Bi, As, Be, B, Mg, Mn, Sm, La, Rh, Ru, Re, Pt or/and Pd; aWe are the atomic ratios of the respective elements; a is 0.01W8; c is 0.01W5; d is 0W20 when b is 12; e is the number of oxygen atoms required to satisfy the valence of the above-mentioned respective components) to keep the reaction system at a low temperature and obtain advantageously the titled compound in sufficient selectivity.