The Ariake Sea has Japan's largest tidal range - up to six meters. Given previous Ariake Sea disasters caused by storm surges and high waves, it is considered highly likely that the bay's innermost coast will be damaged by typhoon-triggered storm surges. Concern with increased storm-surge-related disasters is associated with rising sea levels and increasing typhoon intensity due to global warming. As increasingly more potentially disastrous typhoons cross the area, preventing coastal disasters has become increasingly important. The first step toward doing so is damage prediction, which requires numerical simulation. Our study considers the tracks of typhoons considerably influencing the Ariake Sea. To examine storm-surge risk related to both inundation area and process, we calculated storm surges inundating the Sea's innermost coastal area using an improved ocean-flow finite-volume coastal ocean model. Results showed that enhanced storm surges were to be anticipated and that inundation areas could be extensive where typhoons followed a route from west to northeast across the Sea. We also found that even under current climatic conditions, typhoons able to cause significant storm-surge and inundation disasters could adversely affect the Bay's innermost coastal area. Our analysis of this area and process indicated that the inundation extent around the bay's innermost coast varies with the typhoon, confirming the importance of determining typhoon routes triggering the potentially greatest inundation damage.