It is believed that the abnormal absorption of endotoxin present in the gastrointestinal tract relates to the pathogenesis of autoimmune diseases such as rheumatoid arthritis. In an animal model for rheumatoid arthritis, it is observed that arthritis is improved by removing endotoxin. The present invention provides an endotoxin-adsorbent, which is capable of removing endotoxin in the gastrointestinal tract and can be administered to humans safely. By using a non-digestible and non-absorbable, and therefore, safe endotoxin-adsorbent, which has a high endotoxin-binding capacity for removing large amounts of endotoxin present in the gastrointestinal tract, it is possible to prevent and treat autoimmune diseases such as rheumatoid arthritis.