ALERA is an experimental atmospheric reanalysis dataset for about one and a half years from 1 May 2005 produced on the Earth Simulator. It provides not only the ensemble mean but also spread of the ensemble members. The spread could be used as a measure of the analysis error. This datatset is produced under the collaboration among the Japan Meteorological Agency (JMA), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), and Chiba Institute of Science (CIS). ALERA may be used for research purposes for free under the terms and conditions . AFES (AGCM for the Earth Simulator) is run at a resolution of T159/L48 (about 80-km in the horizontal and 48 layers in the vertical). The ensemble size is chosen to be 40. Observational data excluding satellite radiances are assimillated using the LETKF (local ensemble transform Kalman filter).