A method of producing a sugar liquid from cellulose-containing biomass includes (1) to (4): (1) subjecting a cellulose-containing biomass to a dilute sulfuric acid treatment and thereafter separating the treated cellulose-containing biomass into a dilute sulfuric acid-treated liquid and a cellulose-containing solid content; (2) adding a cellulase to the cellulose-containing solid content to hydrolyze the cellulose and thereafter obtaining a sugar liquid; (3) filtering the dilute sulfuric acid-treated liquid through a nanofiltration membrane at pH 2.5 or lower to thereby separate a sugar concentrated liquid as a retentate and at the same time recover a sulfuric acid aqueous solution as a permeate; and (4) reusing the whole amount or a part of the sulfuric acid aqueous solution obtained in (3) in the dilute sulfuric acid treatment in (1).