Hirohisa Fujii, the new finance minister, says he doesn't support a weak yen. The world's biggest banks say that's just what he may get.

While the yen gained against all but one of the 16 most actively traded currencies since early August as the Democratic Party of Japan became the likely winner in the election, forecasters say it will decline 5.1 percent against the dollar and 0.8 percent versus the euro by yearend.

The economy is too weak to support a stronger rate, based on the median of 40 estimates in a Bloomberg survey.