Fluctuations of subsurface temperature were measured and analyzed to evaluate the activity of a mud volcano in Matsudai district, Niigata prefecture, Japan. Fourteen thermocouples were installed in an inclined observation well and subsurface temperature was continuously measured for ten minutes. Some fluctuations of subsurface temperature over a period of a few days to several weeks were observed at some monitoring points along the well. These fluctuations may be caused by changes of flux and path of subsurface fluid flow. The results of this study suggest that measuring subsurface temperature is effective to monitor long-term changes of mud volcano activity.