RITE is undertaking the Japanese National R&D project "Underground Storage Of CO2", which started in 2000, in cooperation with Engineering Advancement Association of Japan (ENAA). We initiated the carbon dioxide injection in July 2003 for a small-scale field test in South Nagaoka Gas Field. The target aquifer layer with a depth of 1100 in has a thickness of 20 in and the planned total amount of the injection by the end of 2004 is around several thousand tonnes. Our monitoring effort both from surface and in downhole using 4 wells (one-for injection and three for observation), includes: continuous P, T downhole at the injection point continuous aquifer monitoring of P, T in an observation well, downhole logging at 3 observation wells at regular basis, time-lapse crosswell seismic, etc.