In processing of biomass by catalytic cracking in a fluidized catalytic cracker having a reaction zone, a separation zone, a stripping zone, and a regeneration zone, the feedstock oil containing the biomass is processed in the reaction zone using a catalyst containing 10 to 50 mass % of ultrastable Y-type zeolite under the conditions: outlet temperature of the reaction zone 580 to 680° C., catalyst/oil ratio 10 to 40 wt/wt, reaction pressure 1 to 3 kg/cm2 G, and contact time of the feedstock oil with the catalyst in the reaction zone 0.1 to 1.0 sec, and the catalyst is then treated in the regeneration zone under the conditions: regeneration zone temperature 640 to 720° C., regeneration zone pressure 1 to 3 kg/cm2 G, and exhaust gas oxygen concentration at the regeneration zone outlet 0 to 3 mol %.