Excellent water repellency, Oil repellency, stain block property and yellowing-resistance are imparted to a textile by a method of preparing a treated textile has the steps of (1) preparing a treatment liquid containing a water- and oil-repellent agent and a stain blocking agent, (2) bringing a pH of the treatment liquid to at most 7, (3) applying the treatment liquid to the textile, (4) treating the textile with steam, and (5) washing the textile with water and dehydrating the textile. The treatment liquid contains a sulfated fatty acid compound.