A biomass gasification system for producing aqueous or water gases after biomass has been carbonized is disclosed. Temperatures of a thermal decomposition and gasification furnace can be quickly and uniformly stabilized with smaller thermal loss. Reaction residuals after thermal decomposition and gasification are prevented from adhering on the inner surface of the system. The biomass gasification system comprises: a main body, a first cylindrical member, a first cut-out member, a first cylinder accommodating therein a first screw conveyor, a second cylindrical member, a second cut-out member, a second cylinder accommodating therein a second screw conveyor. The first cylinder is so constructed that it penetrates the main body, the first cylindrical member and the first cut-out member in an axial direction. The first screw conveyor, the second screw conveyor and the second cut-out member have a plurality of gasifying agent ports, respectively.