A process for producing a composite oxide catalyst to be used for gas phase oxidation of an olefin or unsaturated aldehyde with molecular oxygen to produce the corresponding unsaturated aldehyde and/or unsaturated carboxylic acid, which has a uniform constant high performance, in an industrial scale and efficiently and constantly, is presented.The process comprises a step of molding a catalyst component-containing powder and a step of calcining a molded product obtained in the molding step, wherein the molding step is a step wherein graphite particles having an average particle diameter D.sub.50 of from 10 to 50 .mu.m and having a combustion initiating temperature in a differential thermogravimetric analysis higher by at least 50.degree. C. than the calcination temperature in the next calcination step, are added to the catalyst component-containing powder in an amount of from 0.5 to 10 wt %, based on the powder, followed by molding, and the calcination step is a step wherein the calcination is carried out at a temperature of at least 250.degree. C. and lower by at least 50.degree. C. than the combustion initiating temperature of the graphite particles.