A method of producing a sugar liquid includes steps (1) providing a pretreatment product of a first cellulose-containing biomass; (2) adding a second cellulose-containing biomass or a pretreatment product thereof in an amount ranging from 1 to 50% based on the solid content weight of the first cellulose-containing biomass pretreatment product of step (1), and carrying out hydrolysis by a filamentous fungus-derived cellulase; and (3) subjecting a hydrolysate of step (2) to solid-liquid separation to obtain a solution component and a hydrolysis residue, and filtering the solution component through an ultrafiltration membrane to recover the filamentous fungus-derived cellulase as a retentate and recover a sugar liquid as a permeate.