It is well known that magnesium enhances the effect of neuromuscular blocking agents such as rocuronium. However, it is not known how much magnesium can reduce neuromuscular blocking agents in maintaining deep neuromuscular blockade. Through this study, the investigators will quantitatively analyze the rocuronium saving effect of magnesium.

It is well known that magnesium potentiate the effect of neuromuscular blocking agents. It prolonged the duration of rocuronium and reduces the onset time of rocuronium. Magnesium acts on motor end plate, where magnesium reduces the release of prejunctional acetylcholine, thereby decreasing the muscle membrane excitability. However, the quantitative rocuronium saving effect of magnesium is not clear. This study's hypothesis is that magnesium would reduce the amount of rocuronium. And the investigators want to evaluate how much magnesium can reduce the amount of rocuronium. Secondary outcome is the effects of magnesium on recovery time, postoperative pain, nausea and vomiting.

Inclusion Criteria:

• Patients who undergo elective robot assisted radical prostatectomy, between 20 and 80 years with a American Society of Anesthesiologists physical status classification of I or II

Exclusion Criteria:

• Conduction abnormalities in preoperative EKG such as Atrioventricular block, sinus pause
• Glomerular filtration rate <60ml/min/1.73m2
• Allergy to rocuronium, sugammadex, magnesium
• Medications that interfered with muscle activity
• Neuromuscular disease
• Hypermagnesemia