Japan's Maritime Self-Defence Force is only allowed to deploy defensively
Japanese Defence Minister Yasukazu Hamada has ordered the dispatch of ships to fight pirates off Somalia.
He did not say how many ships would be sent or when, but told the military to assess what sort of force was needed.
Japan's parliament and Prime Minister Taro Aso would need to pass a formal order before the ships could set off, a process expected to take a month.
Japan would be joining a multi-national effort against piracy which includes ships from the United States and China.
"The pirates' activities off the Somali coast are a major threat not only to Japan but also to international society and it is a problem that we must deal with urgently," Mr Hamada said.
No Japanese ship has been taken by pirates yet, the government said, but pirates have fired at three Japanese vessels. No-one was injured.
Japan's decision to go ahead with a deployment took months of debate.
The activities of Japan's military are highly restricted by Japan's post-World War II constitution, which limits Japan to conducting only defensive military operations.
World leaders have called for more action to tackle piracy
Ruling party members have argued that battling pirates should be seen as fighting crime on the high seas, not strictly as a military operation.
China announced in late December that it would be sending two navy destroyers and a supply vessel to the Gulf of Aden.
Their main mission was described as protecting Chinese vessels and ships delivering humanitarian aid.
China's deployment was a first for a country that usually eschews involvement in the affairs of other countries.
Also from Asia are ships from South Korea, joining more than a dozen ships from Britain, Iran, the US, France and Germany.
The United Nations Security Council decided in early December to extend for another year its authorisation for countries to enter Somalia's territorial waters, with advance notice, and use "all necessary means" to stop acts of piracy and armed robbery at sea.
Piracy has taken an increasing toll on international shipping, especially in the Gulf of Aden, one of the world's busiest sea lanes.