Takeshi Fujimaki, a former adviser to billionaire investor George Soros who won a seat in the Upper House election last month, said a delay in increasing the sales tax and a reduction in the U.S. Federal Reserve's stimulus could cause the Japanese government bond "bubble" to burst.

Prime Minister Shinzo Abe's administration plans to release later this year its medium-term fiscal plans and a final decision on a two-step doubling of the consumption levy to 10 percent in 2015. Fed Chairman Ben S. Bernanke said in June the U.S. central bank may start scaling down its third round of quantitative easing this year and end the asset purchases altogether in mid-2014.

Japan's public debt, the highest ratio globally, may balloon to 245 percent of gross domestic product this year, according to the International Monetary Fund. Even as the nation struggles to turn its primary balance into a surplus, it enjoys the world's lowest borrowing costs as record bond buying by the Bank of Japan helps keeps yields anchored.