A method produces a concentrated aqueous sugar solution using a cellulose-containing biomass as a raw material, including: (1) hydrolyzing a cellulose-containing biomass to produce an aqueous sugar solution; (2) filtering the aqueous sugar solution obtained in (1) through a microfiltration membrane and/or an ultrafiltration membrane, and recovering an aqueous sugar solution from the permeate side; and (3) filtering the aqueous sugar solution obtained in (2) through a reverse osmosis membrane, recovering a permeate from the permeate side and recovering a concentrated aqueous sugar solution from the feed side; wherein at least a part of the permeate from the reverse osmosis membrane is used as a hydrothermal treatment liquid, biomass-suspending, liquid, enzyme-diluting liquid, acid-diluting liquid and/or alkali-diluting liquid in (1).