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.