Kuchinoerabujima, located in southwestern Japan, is an andesitic volcano where explosive eruptions have repeatedly occurred. Seismicity increased in 1996 and 1999, and inflation of the volcanic body was detected by GPS surveys during the period from 1995/96 to 2000 (Iguchi et al., 2002). We established a continuous GPS observation network in April, 2004 to study the relation between seismicity and ground deformation. Vertical component of a baseline with a particularly large elevation difference is influenced by water vapor in the atmosphere. In this study, a simple atmospheric correction method is proposed to detect an upheaval component of 1 cm order. After the correction, ground deformation starting at the beginning of January, 2005 is clearly recognized at an observation site near the summit crater. The ground deformation has progressed at a rate of about 1 cm per 100 days, and it may be caused by a presumed pressure source at a depth of 300m beneath the summit. The deformation corresponded to increase in seismicity of high-frequency events at depths shallower than 500m. It is inferred that these phenomena were caused by hydrothermal activity.