One of the lessons learned from the Fukushima Daiichi Nuclear Power Plant accident is the importance of identifying plant vulnerability through probabilistic risk assessment (PRA) that consider combinations of multiple external hazards, such as earthquakes and tsunamis. Although studies on probabilistic assessment methodologies for combined hazard scenarios have progressed in recent years, the treatment of such hazards and associated evaluation procedures have not yet been fully standardized. In response to these challenges, JAEA has been conducting research and development efforts to standardize PRA implementation procedures for combined external hazards. This presentation outlines the fundamental concept for standardizing PRA procedures for combined external hazards and discusses the requirements to ensure the completeness of hazard combination and the practical feasibility of screening hazard combinations, particularly in light of the large number of possible combinations.