A method of producing cellulase includes steps (1) to (3): (1) subjecting an aqueous solution of cellulase derived from filamentous fungi to filtration through an ultrafiltration membrane with a molecular weight cut off of 100,000 to 200,000 to obtain a filtrate and concurrently obtain a concentrated enzyme liquid as a retentate; (2) further subjecting the filtrate obtained in step (1) to filtration through a second ultrafiltration membrane with a molecular weight cut off of 5,000 to 50,000 to obtain a second concentrated enzyme liquid as a retentate; and (3) mixing the concentrated enzyme liquid obtained in steps (1) and (2) to obtain cellulase derived from filamentous fungi.