To provide a method and an apparatus for gasifying a biomass with which useful, good quality synthesis gas can be produced efficiently by a small sized apparatus.

The heating of a heat-holding material 54 and the ignition of a biomass fuel 12 are carried out with a same burning nozzle 13; the biomass fuel 12 is burned in a thermal cracking zone of a combustion gasification furnace cylinder 10 to gasify it through thermal cracking; the thermally cracked gas is sucked into a combustion layer zone located on a lower position, and it is reacted with a high temperature mixed gas of steam and air to be oxidized; further, the oxidized gas is treated in a reduction layer and a reforming layer to be converted to reformed gas; and high-quality refined gas is obtained by removing ashes and unreacted tar from the reformed gas. Further, after the refined gas is obtained, steam and air fed to the refining layer zone are heated by using the latent heat of the refined gas, and thus energy is effectively used.