With their tidy suburban home here, a late-model Toyota in the driveway and two school-age children whose Japanese is indistinguishable from any native's, Akio Nakashima and his wife, Yoshie, are the perfect immigrants.
Though Vietnamese by origin, as fellow Asians they would be hard to pick out in a crowd. Through years of diligent study they have mastered this country's difficult language. They even adopted Japanese names.
Outside the workplace, though, in 21 years in this country, where they arrived as boat people in 1982, the Nakashimas have never managed to make friends. Even that is a petty concern compared with the worry that troubles their sleep.
''As far as my life goes, it doesn't matter if I am Vietnamese or Japanese,'' said Mr. Nakashima, 36, an engineer at a tire factory. ''My biggest worry is prejudice and discrimination against my children. We pay the same taxes as anyone else, but will our children be able to work for a big company, or get jobs as civil servants?''
Many economists and demographers here and abroad say Japan's success or failure in addressing the concerns of immigrants like the Nakashimas will go a long way toward determining whether this country remains an economic powerhouse or whether its population shrivels and the slow fade of its economy turns into a rout.
Japan is at the leading edge of a phenomenon that is beginning to strike many advanced countries: rapidly aging populations and dwindling fertility. The size of this country's work force peaked in 1998 and has since entered a decline that experts expect to accelerate.
By midcentury, demographers say, Japan will have 30 percent fewer people, and one million 100-year-olds. By then, 800,000 more people will die each year than are born. By century's end, the United Nations estimates, the present population of 120 million will be cut in half.
Better integration of women into the workplace may help in the short term, but experts say the only hope for stabilizing the population is large-scale immigration, sustained over many years.
Failing that, the consequences could include not only a scarcity of workers and falling demand, but also a collapse of the pension system as the tax base shrinks and the elderly population booms.
To stave off such a disaster, Japan would need 17 million new immigrants by 2050, according to a recent United Nations report. Other estimates have said Japan would need 400,000 new immigrants each year.
But Japan is the most tenaciously insular of all the world's top industrial countries, and deeply conservative notions about ethnic purity make it hard for even the experts here to envision large-scale immigration.
Seventeen million immigrants, as the United Nations forecasts, would represent 18 percent of the population in a country where immigrants now amount to only one percent.
Even that modest figure consists mostly of second- and third-generation Koreans and Chinese whose ancestors were brought to Japan when it maintained colonies on the Asian mainland. As the Nakashimas, from Vietnam, know all too well, even long-term immigrants face frequent discrimination and are not accepted as ''real'' Japanese.
''The kind of figures the demographers talk about are unimaginable for Japan,'' said Hiroshi Komai, a population expert at Tsukuba University. ''In a quarter-century we have only absorbed one million immigrants.
''Societies have always risen and faded, and Japan will likely disappear and something else will take its place, but that's not such a problem. Greece and Rome disappeared too.''
