A seismic zoning map for the building standard law in Japan was first set up in 1952 taking account of not only historical seismic activity but also political wisdom. Development of this kind of map was so slow that nearly the same map has been adopted to a newest building regulation. There was no remarkable study to stimulate the political wisdom. A cost-benefit model is employed to investigate adequate seismic design coefficients for buildings to withstand destructive earthquakes. The model consists of a profit for rent, a charge for initial and maintenance costs and a damage loss due to collapse in a unit area of a building. The initial and maintenance costs are functions of a design acceleration. It is assumed that building collapse occurs when ground acceleration exceeds a specific design acceleration. The probabilities of ground acceleration at four sites in Japan are taken from a report investigated by a probabilistic analysis base on a Poisson' s process of earthquakes and Gutenberg-Richiter's law. The adequate level of design acceleration may be determined from the level at which the maximum net profit is given. For seismic zoning factors the parameters of the model are modified with the economical factor of the sites. Obtained results are: 1) The adequate level of the design earthquake is at about a 90 percent probability of not being exceeded in a 50 year interval. 2) Net profits are not sensitive near the adequate level so that over strength design is rather recommendable than underestimated design to avoid unexpected severe damage. 3) The seimic zoning factors currently used in Japan for buildings are adequate if the design level in Tokyo is adequate, otherwise are questionable.