HE Group has produced phthalic anhydride at its Chiba plant since 1979 by using a fluidized bed reactor. A ground silica-based catalyst was used at the beginning of fluidized bed operation. By changing to a spherical type catalyst, the duration of catalytic activity was increased and catalyst density in the bed was reduced by around 10%. Addition of Cs to the catalyst enhanced its activity. Non-active silica gel was used as diluent for matching catalyst activity with plant capacity. The active component of Cs added to the catalyst has a lower melting point, the catalyst particles bound with it. The fluidized bed was operated stably by controlling the Cs/K ratio.