In the event of disaster, the risk of disaster are intertwined, and there is an occurrence possibility of simultaneous damage in multiple areas. Nationwide companies have more risks of simultaneous damage in multiple areas by one disaster. For example, factories in Osaka and in Nagoya, can be damaged by one typhoon. In this case, company will need more money when damage happened and better to make special insurance contract, e.g. Catastrophe bond. On the other hand, insurance company has to assess amount of insurance payout because to pay it for contracted companies quickly. Insurance company may have difficulty to estimate total amount since there are few researches assessing aggregate loss caused by coastal disasters. This research proposes a procedure of assessment of aggregate loss by storm surges in Ise and Mikawa Bay located in Aichi prefecture, Japan.