Japan's current account surplus - the difference between the export and import of goods and services - rose in December for the fourth straight month.
Japan's automotive exports are buoyant
It grew by 8.6% to 1.7 trillion yen ($14.8bn; £8.5bn) from a year earlier.
The Finance Ministry reported a rise in both exports and imports, another sign of Japan's sustained economic recovery.
The trade surplus fell as imports jumped by 30% and exports by 19%, but this was offset by a surge in income from companies' overseas investments.
Much of the investment income came from US and European bonds - Japan is one of the world's biggest buyers of US government debt.
The big rise in the cost of imports was mainly due to high oil and commodity prices.
Export growth was particularly strong for the automotive and technology industries, with demand rising in Japan's key markets of China and the US.
"Overall, the latest balance of payments data can be interpreted as a sign that the Japanese economy remains on track towards steady and moderate growth," said Hiroaki Muto, senior economist at Sumitomo Mitsui Asset Management.
The figures are likely to reinforce market expectations that the Bank of Japan will raise interest rates within the next few months as the economy recovers.
Overall, Japan's current account surplus for the whole of 2005 stood at 18 trillion yen, down 3% on the year before.
The annual trade surplus fell 25% to 10 trillion yen as an 18% rise in imports overtook a 7% rise in exports.