To provide a biomass gasification power generation system efficiently producing a fuel gas of methane, hydrogen or the like by gasifying biomass by supercritical water while efficiently utilizing energy, and supplying electric power generated by the obtained fuel gas.

This biomass gasification power generation system comprises: a pretreatment device for preparing slurry of biomass; a reactor for gasifying a slurry member of biomass; a power generater for generating power while applying the produced gas produced by the reactor as a fuel; and a heater for heating the reactor by burning a part of the produced gas in a gas including oxygen. A system obtained by further providing the system with a preheater for preheating the gas including oxygen by utilizing the heat of an exhaust gas produced by burning the produced gas in the heater, comprises; a preheater for preheating the gas including oxygen used in the heater by utilizing the heat of the exhaust gas discharged from the power generater; or a means for heating the biomass in a pressure hydrothermal liquefaction reactor.