We demonstrate the preventive effect of bittern water (BW), which enables the effective intake of magnesium ion (Mg²⁺), on paw edema in adjuvant-induced arthritis (AA) rat. BW (five kinds; BW-1, 2, 3, 4, 5) containing 10—200 mg/l Mg²⁺ was used in this study. Arthritis was induced by the injection of 50 μl of a suspension of 10 mg/ml heat-killed butyricum (Mycobacterium butyricum) in Bayol F oil into the plantar region of the right hind foot and tail of rats. Paw edema of the right and left hind feet of AA rats were reduced by the administration of BW for 14 d after adjuvant injection in comparison with those of AA rats administered purified water. The preventive effect increased with the increasing Mg²⁺ content of the BW. In addition, a combination of indomethacin (IM, 2 mg/kg) and BW-5 (200 mg/l Mg²⁺) prevented paw edema of the right and left hind feet of AA rats in comparison with IM alone. The fate of plasma IM after the oral administration of the combined IM (2 mg/kg/d) and BW-5 was similar to that after the administration of IM alone. In conclusion, the oral administration of Mg²⁺ to AA rats potently prevents the development of inflammation, and the combination of IM and Mg²⁺ may provide an effective therapy of arthritic edema.