The Geological Survey of Japan, AIST, has been monitoring groundwater in and around the Kinki and Tokai districts for earthquake prediction research. The Niigata Prefectural Office has also been observing groundwater for monitoring land subsidence in Niigata Prefecture. The 2004 Niigata-Chuetsu (M(JMA) 6.8) and 2007 Chuetsuoki (M(JMA) 6.8) earthquakes occurred in Niigata Prefecture, Japan, on October 23, 2004 and July 16, 2007, respectively. The two earthquakes have a similar magnitude, epicenter, and mechanism. At many of the observation wells, we detected changes in groundwater level or pressure related to the two earthquakes, but no clear precursory changes. At all of our observation wells in Niigata Prefecture, trend changes were observed after coseismic step-like changes for both of the earthquakes. At some of the Stations in and around the Kinki and Tokai districts, coseismic trend changes and/or step-like changes were observed. The pattern of the changes were almost similar for the two earthquakes. Those changes were considered to be caused not by the static crustal deformation but by the ground shaking.