Recently expanding market for largely scale construction projects, cost management of construction project have been increasingly recognized as an important factor responsible for the project success. But public organizations have been continuously operating the same methods as before in cost estimating, so it raise an issue of reliability for estimated cost. Therefore, this study present a highly reliable model that makes preliminary estimates based quantity data in early design stage. it enables engineers to efficiently manage total cost of project as well as partial cost estimating. This study develop a model of database that is created with historical data conducted by government offices. The model proposed can estirnate a cost for RC Structure in schematic design stage, and it is applied to CBR+GA logics to explore best similar cases. In case of estimating methods used to previous research, it is based on actual cost data. Due to the cost estimating compensated by inflation rate vary in market economic conditions, it will follow as an inevitable consequence of reliability problem. It needs to find better way to ensure accuracy through quantity database. Also, This study has been conducted in attempts to verify the reliability of preliminary estirnation. It is checked for the feasibility of presented model with P-National Tax Service Project aside from the database.