We observed the phenomena occurring in a fluidized catalyst bed by using a reactor made of transparent glass in order to study the decrease in fluidization quality during a reaction involving a decrease in gas volume. It was found that the upper part of the bed agglomerated with the progress of the reaction, and defluidization occurred. This agglomerated part was lifted up by the fluidizing gases passing through the column. Since the difference in density between the gases in the bubble and emulsion phases affected the fluidization behavior, we studied the effect of the gas density on the emulsion-phase contraction. In the present study, this effect was found to be insignificant. The decrease in fluidization quality caused by the reaction involving gas volume reduction was considered to be due to the decrease in gas volume in the emulsion phase and the agglomeration of the bed, which prevented the formation and rise of bubbles in the bed.