Abstract: According to the method of the present invention, a cellulosic biomass slurry whose concentration of cellulosic biomass between 1% and 5% by mass is hydrothermally treated at a temperature of between 140° C. and 200° C. a pressure of between 1 MPa and 5 MPa to saccharify/decompose hemicellulose into C5 sugars. Then, a dewatered cake obtained after the hydrothermal treatment is slurried and has a solid concentration of between 1% and 5% by mass, and the slurry is hydrothermally treated at a temperature of between 240° C. and 300° C. and a pressure of between 4 MPa and 10 MPa to saccharify/decompose cellulose into C6 sugars. A saccharified solution is concentrated by a concentration device such as a reverse osmosis membrane device so that the concentration of sugars is 10% by mass or higher, and is then subjected to alcoholic fermentation.