In a process for producing an antimony-containing oxide catalyst supported on silica for use in a fluidized bed reaction by the steps of preparing a slurry containing an antimony compound, a polyvalent metal compound, and a silica sol as the essential ingredients, heat treating the slurry at a pH of 7 or less and at a temperature of 40.degree. C. or higher, and thereafter drying the slurry and calcining the dried particles, the improvement wherein said silica sol has a multi-peak particle size distribution profile that is derived by using a mixture of silica sol A in an amount corresponding to from 10 to 90 wt% of the total silica in the catalyst and silica sol B in an amount corresponding to 90 to 10 wt% of the total silica in the catalyst, with silica sols A and B having different average particle sizes, provided that the relations 3<da<100 and 0.1<da/db<0.9 are satisfied wherein da (m.mu.) is the average particle size of silica sol A, and db (m.mu.) is the average particle size of silica sol B.