Method for producing composite oxide catalyst

A method for producing a composite oxide catalyst, which is a method for producing a composite oxide catalyst having the following formula (I), to be used at the time of gas phase catalytic oxidation of an unsaturated aldehyde with a molecular oxygen-containing gas to produce the corresponding unsaturated carboxylic acid, characterized in that Sb2O3 of isometric system is used as at least a part of an antimony-supplying source compound:

• Mo12XaVbSbcCudSieCfOg (I)
• where the respective components and variables have the following meanings:
• X is at least one element selected from the group consisting of Nb and W;
• a, b, c, d, e, f and g represent atomic ratios of the respective elements;
• against 12 of molybdenum atom 0<a≦10, 0<b≦10, 0<c≦5, 0<d≦5, 0≦e≦1,000 and 0≦f≦1,000;
• and
• g is a number determined by the degrees of oxidation of the above respective components, wherein
• Sb2O3 is used as the antimony-supplying source compound;
• and
• the ratio (a1/a2) of the intensity (a1) at 2θ=about 28.3° to the intensity (a2) at 2θ=about 27.6° of an X-ray diffraction (anticathode Cu—Kα) of the antimony-supplying source compound is at most 0.2.