A 68-year-old man with a 14-month history of dialysis was admitted to our hospital with a chief presentation of dysphagia. The next day, dyspnea developed, and the patient was intubated. On the fourth day of admission, tonic spasms and posterior arch retraction were observed. Tetanus was diagnosed, midazolam was administered, and as the spasms were uncontrollable, magnesium sulfate was administered repeatedly while checking the blood magnesium level, to achieve a target level of 2.5-3.0 mEq/L. Continuous infusion was not performed because of hemodialysis, and the seizures decreased as the blood concentration of magnesium increased. On the thirteenth day of hospitalization, bradycardia was observed, which increased the blood magnesium level to 2.9 mEq/L, and magnesium administration was discontinued. The patient weighed 70 kg and was given a total of 240 mEq12A intermittently, with an average increase in the blood concentration of 0.33 mEq/L per 20 mEq. Magnesium has recently been administered to treat tetanus； however, an appropriate dosage for patients undergoing dialysis remains unknown. In summary, we report a case in which magnesium sulfate was administered to a patient on hemodialysis who developed tetanus, and throughout treatment, his magnesium levels remained high.