Table 3 shows international comparison of the speed of population ageing among seven industrialized countries with respect to the year attained or expected to attain the 7, 10 and 14 percent level in terms of the proportion of the aged. Particularly, the last column indicates number of years required to shift from 7 percent to 14 percent. As can be noted in this column, the rapidity of population ageing in Japan is very impressive. The number of years expected to spend for moving from 7 to 14 percent would take only 25 years in Japan and is distinctively shorter than any other selected developed countries. When the speed of ageing is rapid, its social and economic impacts are much greater than otherwise in the situation where the change takes place rather showly. Hence, people and society's response to those demographic changes must be more difficult in Japan, requiring swift adjustment to those changes and restructuring its institutional setup and infrastructure.
Why is the speed of population ageing so rapid in Japan? This point will be discussed in detail in the next section, but this is because the speed of fertility decline in the postwar years was so rapid in Japan. Table 4 shows the trends of fertility in Japan. It is remarkable that crude birth rate which was 34.3 per 1000 population in 1947 had been halved to 17.2 in 1957, there-after fertility never coming back to the level higher than 20 per 1000. The almost unprecedented rapidity of birth rate decline and the continuity of very low fertility level are the principal causes for presenting Japan as a country of the most rapidly ageing population in the world.
Source： Before 1940： United Nations, The Aging of Population and its Economicand Social Implication.
Population Studies, No.26, New York, 1956.
After 1940： United Nations, The Sex and Age Distribution of World Population, 1996 New York, 1997.
