This method of upgrading a biomass comprises: an upgrading step for performing upgrading treatment of a cellulose based biomass with an oxygen/carbon atomic ratio of at least 0.5, in presence of water and under a pressure of at least saturated water vapor pressure, and reducing said oxygen/carbon atomic ratio of said biomass to no more than 0.38, and a separation step for separating an upgraded reactant obtained from said upgrading step into a solid component and a liquid component.