An advance contingent of Japanese military personnel has set off on a humanitarian mission to Iraq.
Japan's decision to send troops has provoked domestic controversy
The first party of about 20 air force personnel left Tokyo on two commercial flights on Friday morning.
Japan plans to send about 1,000 troops to aid reconstruction efforts in the country's biggest foreign deployment since World War II.
Japan's constitution bans its soldiers from fighting overseas, and the mission has raised opposition at home.
A special law allowing the deployment was passed by Japan's parliament in July, but only under the condition that the troops be sent to a place away from combat.
The Japanese cabinet approved sending the force on 9 December.
Japan's pacifist constitution, adopted after World War II, forbids the country from using force to settle disputes.
The Japanese public fears its troops will be drawn into the Iraq conflict
The air force personnel who left on Friday are part of a 40-strong advance team who are travelling to Kuwait and Qatar to prepare for the arrival of a larger unit in January.
Japan opted not to use its own military or government planes, but sent them on British Airways commercial flights instead.
Most of the troops will be based in south-eastern Iraq, where they will help restore water services, offer medical aid and rebuild schools.
Japan's Chief Cabinet Secretary Yasuo Fukuda told a news conference that the troops would make "a great contribution" to reconstruction efforts in Iraq.
At a ceremony to mark the formation of the air force's contingent on Wednesday, Japanese Prime Minister Junichiro Koizumi said Japan had a responsibility to help restore
stability to Iraq.
"Japan's prosperity and development rests on the peace and stability of the world," he was quoted by the Associated Press news agency as saying.
"We cannot be absorbed in ourselves. We must lend our strength for the reconstruction and peace of other countries."
However, recent opinion polls have suggested that up to two-thirds of the Japanese public are opposed to sending military personnel to Iraq.
Incidents such as the suicide bombing at the UN office in Baghdad in August and the killing of two Japanese diplomats on 29 November have added to public concern.