The supplementary motor cortex (SMA) is located anterior to the primary motor cortex, and is considered to play an important role in planning, initiating and maintaining sequential motor actions. Disturbance of this area sometimes causes severe contralateral hemiplegia. If the disturbance doesn't affect the primary motor cortex, motor function will recover in relatively early postoperative time. We encountered two cases in which the patients developed postoperative severe hemiplegia after resection of a medial frontal lobe glioma, although there was no apparent change shown in the monitoring of intraoperative motor evoked potential (MEP). Both cases recovered from hemiplegia in the early postoperative period. In our cases, the disturbances of SMA were considered to be the causes of the development of transient hemiplegia. Intraoperative MEP monitoring is useful to distinguish the damage of the primary motor area from that of the SMA.