Japan's current account surplus shrank nearly a fifth to a record low $22 billion in 2014, the fourth straight annual fall, as its trade deficit swelled amid a weak yen, official data showed Monday.

The surplus on its current account fell from 3.23 trillion yen in 2013 to 2.63 trillion yen last year, the smallest since 1985, when comparable data became available, according to the finance ministry.

The current account is the broadest measure of the country's trade with the rest of the world, including not only trade in goods but also services, tourism and returns on the country's foreign investment.

Japan's trade deficit last year grew by 18.1 percent to 1.04 trillion yen from the previous year, as the cost of imports of oil and gas -- which is priced in dollars -- overwhelmed export growth.

Japan has been saddled with a trade imbalance stoked by its heavy dependence on importing fossil fuels to generate electricity, with nuclear reactors shut down in response to the 2011 tsunami-sparked atomic disaster.

But overall income is improving with higher gains from equity and other direct investment, as well as from investment in financial items.

The rise has been inflated by a weaker yen, the consequence of Prime Minister Shinzo Abe's pro-spending policy and the Bank of Japan's massive monetary easing.

In December alone Japan posted a current account surplus of 187.2 billion yen, the sixth straight monthly surplus, reversing a deficit of 679.9 billion yen a year earlier.

Capital Economics said Japan's current account surplus should continue to improve in coming months as the trade balance may return to surplus as the oil price falls.

"Looking ahead, we expect the yen to weaken towards 140 against the dollar by the end of the year, which should provide an additional boost to the income balance," it said.

"What's more, the plunge in the price of crude oil since last summer has yet to feed through to import prices in full," it added. "Once this happens, the trade balance may briefly return to surplus."