Gasification of sawdust was conducted using an experimental fluidized bed with inert particles under the differential and stable condition, by feeding the dried sawdust particles. The gas production rates were measured under the conditions with steam, f(c)(1) and without steam, f(c), [mol-C/s]. The overall gasification reaction ra constant, k [1/s], was determined from the difference in production rate of gaseous carbon between the conditions with steam and without steam [g-C/s], divided by and weight of carbon in bed for various partial pressures of steam, P-H2O, temperatures, T [K]. By the present method, the rate was measured under the actual condition of fluidized bed biomass gasifiers; rapid carbonization of sawdust, and simultaneous pyrolysis and gasification. The rate was given as a function of T and P-H2O. Finally, the observed overall rate constant in the bed was related with the dynamic change in rate constant with the progress in conversion of char, which are usually obtained by batchwise experiments, such as TGA.