In this paper we introduce our researches on development of a risk assessment tool for geological CCS (Carbon Dioxide Capture and Storage) that covers geological strata, marine environments, ground surface, ambient air and injection site and its vicinity. To obtain risk values in mentioning area: 1) we have been analyzing CO2 migrations in relation with various geological properties including flow rate of aquifers, faults, by laboratory experiments and numerical simulations. 2) We also have been analyzing dispersion of CO2 from ground surface to ambient air using ADMER 2.5 simulation program. 3) Our analysis about marine environments has just started. Diffusion of CO2 and heavy metals, and their impact on benthos and marine planktons is going to be analyzed by experimental and simulation studies. 4) As regard with risks in ground surface, we are evaluating CO2 related safety risk levels of injection facilities by analogical industrial accident statistics including high-pressure gas laboratories. We are developing risk assessment tool, named GERAS-CO2GS (Geo-environmental Risk Assessment System, CO2 Geological Storage Risk Assessment System), to analyze risks in relation with migration of injected CO2, and to assist safety and risk management. At this moment, GERAS-CO2GS calculates CO2 retention and leakage of geological strata models. We are going to add some risk scenarios including near ground surface and sea, so that the system will evaluate total risks of geological CCS. We are going to combine results of above-mentioned researches into GERAS- CO2GS program accordingly. On the other hand, risk data will be stored in GERAS-CO2GS categorized by endpoints. It is expected that development of GERAS-CO2GS will contribute to risk assessment of the individual injection sites. It will facilitate understanding of risks around geological CCS by legislators and peoples around injection site.