Abstract: An object of the present invention is to provide an ion-exchange membrane for simply and inexpensively separating and purifying exosomes present in a biological sample such as serum. The invention relates to a cellulose-based ion-exchange membrane containing a cellulose-based polymer having at least one hydroxyl group or acetyl group at the 2-, 3-, or 6-position being replaced with a positively charged compound. The invention also relates to a method for purifying exosomes, including subjecting a sample containing exosomes to membrane permeation by using the cellulose-based ion-exchange membrane to allow for adsorption of the exosomes, bringing the membrane into contact with a washing liquid to remove impurities, and bringing the membrane into contact with an eluent to allow for desorption of the exosomes.