The maintenance of partial neuromuscular blocking during general anesthesia for neurosurgery is essential for intraoperative motor-evoked potential monitoring. However, the precise strategy of administering neuromuscular blocking agent for obtaining that goal has not been established. Therefore, the investigators tried to find the optimal initial dose of vecuronium infusion and determine the adequate goal of neuromuscular blocking as guided by neuromuscular transmission module (M-NMT Module, Datex-Ohmeda Inc, Helsinki, Finland).