A patient developed prolonged apnea after emergency cesarean section. The patient was a 38-year-old primiparous woman with myoma uteri and her body weight was 44.8 kg. She received intravenous magnesium sulfate 1 g x hr(-1) as tocolytic agent for threatened premature delivery. We performed rapid sequence induction of anesthesia with thiopental 250 mg and rocuronium 50 mg because she was suspected of full stomach. After the intubation, we did not administer any muscle relaxants during the operation. After the operation which was finished in 65 minutes, the effect of rocuronium remained for more than 100 minutes after its administration probably due to hypermagnesemia. Rocuronium shows rapid onset, and several studies indicate that it can be used for rapid sequence induction instead of suxamethonium. But rocuronium should be used carefully and its effect should be monitored in a patient with hypermagnesemia.