Floating offshore wind turbines (FOWTs) are now at the stage of demonstration experiments in Japan. They have a specific risk of “drift”, which is negligible for bottom-fixed offshore wind turbines. Taking account of the risk of drift, it is necessary to consider the spatial relations between a FOWT and surrounding sea-lanes, coastal facilities, and ecosystem, in addition to the profitability and stakes in fishing industry, when selecting an appropriate site for installing the FOWT. In this paper, the authors propose the method of site selection for installing a FOWT by quantitative risk assessment, taking account of the spatial relations between the FOWT and surrounding sea-lanes, coastal facilities and ecosystem. Based on the calculation results in the case study of the quantitative risk assessment, the following procedure is recommended: first, extract the area where the risks related to the sea-lane are small enough, and second, select the position within the extracted area with the smallest risk of collision with coastal facilities or ecosystem in case of drift of the FOWT by quantitative evaluation of the risk distribution under the environmental conditions in the area. The proposed procedure is commonly applicable in most areas of Japanese coast.