A method for producing a composite oxide catalyst for gas phase catalytic oxidation of an unsaturated aldehyde with a molecular oxygen-containing gas to produce the corresponding unsaturated carboxylic acid in good yield, is presented. A method for producing a composite oxide catalyst, which is a method for producing a composite oxide catalyst having the following formula (I), characterized in that Sb.sub.2O.sub.3 of isometric system is used as at least a part of an antimony-supplying source compound: Mo.sub.12X.sub.aV.sub.bSb.sub.cCu.sub.dSi.sub.eC.sub.fO.sub.g (I) (wherein 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, and against 12 of molybdenum atom, 0