A biomass gasification power generation system including: a gas generation device that generates a combustible gas from a biomass and a gasification agent; an internal combustion engine that generates motive power from a fuel gas containing the combustible gas generated by the gas generation device; and a generator that generates electric power from the motive power generated by the internal combustion engine. The power generation system is additionally provided with a water electrolysis device that generates oxygen and hydrogen through the electrolysis of water. The gasification agent contains oxygen generated by the water electrolysis device, and the fuel gas contains hydrogen generated by the water electrolysis device. The oxygen concentration in the gasification agent is from 22 vol. % to 40 vol. %.