Several key questions need to be answered when CO2 geological storage is to be undertaken worldwide. How should CO2 be stored underground? Can trapping be assumed in saline formations and can CO2 be retained for long periods safely in the subsurface? The first Japanese pilot-scale CO2 sequestration project in Nagaoka was undertaken to provide answers to these questions. The injection site is located at the Minami-Nagaoka gas field in Nagaoka City, 200km north of Tokyo. Supercritical CO2 was injected into an onshore saline aquifer at a depth of 1,100m. CO2 was injected at a rate of 20 to 40 tonnes per day over an 18-month period, with a cumulative amount of 10,400 tonnes. A series of monitoring activities, which consisted of time-lapse well logging, crosswell seismic tomography, 3D seismic survey and formation fluid sampling, was carried out successfully to monitor CO2 movement in the sandstone reservoir. This paper presents an overview of the results obtained from both field and laboratory studies to examine the spatial-time distribution of CO2 and various trapping mechanisms in the reservoir. CO2 breakthrough at two of the three observation wells was clearly identified by changes in resistivity, sonic P-wave velocity and neutron porosity from time-lapse well logging. Each velocity difference tomogram obtained by crosswell seismic tomography showed a striking anomaly area around the injection well. As the amount of injected CO2 increased, the low-velocity zone expanded preferentially along the formation up-dip direction during the first two monitoring surveys and less change around the CO2-bearing zone could be confirmed from the following surveys. Unfortunately there was no significant change in 3D seismic results due to CO2 injection. The pilot-scale project demonstrated that CO2 can be injected into a deep saline aquifer without adverse health, safety or environmental effects. The Nagaoka project also provides unique data to develop economically viable, environmentally effective options for reducing carbon emissions in Japan.