There are about 10 million existing non-conforming wooden houses today in Japan, and upgrading the seismic performance of such houses is essential for disaster mitigation. However, most of them remain untouched because of large expenditure for upgrading for both the owners and society. The owners have to spend about 1.1 million yens on average to satisfy the current design requirement. In Aichi prefecture alone, it would cost about one trillion yens, which is more than 1/3 of the annual budget, to upgrade all of them. In order to implement the upgrading of existing non-conforming wooden houses more efficiently, this paper investigates the alternative strategies for upgrading. The seismic risks of wooden houses as a whole in Aichi prefecture is estimated and then the effective target level for upgrading is discussed from the viewpoint of both economic loss and the number of fatalities.
