The present invention provides a method for producing a useful substance efficiently from cellulose without using any cellulase preparation. According to the disclosures of the present specification, in the production of a useful substance from a cellulose-containing material, the cellulose-containing material is brought into contact with an ionic liquid to cause the ionic liquid to permeate the cellulose-containing material, and a carbon source comprising the cellulose in the cellulose-containing material is simultaneously saccharified and fermented by a cellulase-producing microorganism in the presence of the ionic liquid.