To increase the calorific value of the gaseous product from the woody biomass gasification process, a circulating dual-bubbling fluidized-bed gasification system based on the concept of physical separation of the combustion zone from the gasification zone has been proposed. To cope with the difficulty in control of the tar behavior, a catalytic porous gamma-alumina has been considered as a bed material for the above process, which circulates between the biomass pyrolysis/steam gasification zone and the char/coke combustion zone, capturing tar vapor as coke on its surface catalytically in the gasifier. Preliminary process simulation clarified the minimum amount of carbonaceous portion (char and coke) in the biomass that must be burnt in the combustor for a thermally self-sustained process operation, suggesting that the coke derived from tar deposited over gamma-alumina should be distributed to combustion and gasification zones by controlling the extent of the steam gasification of coke. To develop a method to control the tar conversion during pyrolysis and gasification, we examined the effects of porous gamma-alumina on the tar behavior in the secondary reaction in a two-stage bubbling fluidized-bed reactor. Both gasification tests and Brunauer-Emmett-Teller analyses revealed that porous gamma-alumina was markedly catalytic toward coking and dehydrogenation of the tar vapor, until a substantial amount of coke had deposited on the catalyst, thus occluding pores.