Japan's defence agency has formally ordered the dispatch of 600 troops to help rebuild Iraq's infrastructure.
Advance troops have been warmly welcomed in the town of Samawah
The first soldiers will leave around 3 February, JiJi Press reported, joining a handful of troops already there.
The final green light to go was conditional on this advance party's report on safety conditions.
The move is controversial because critics argue it could violate Japan's pacifist constitution if the troops are drawn into combat.
The defence agency's approval was issued after Japanese Prime Minister Junichiro Koizumi won the backing of his junior coalition partner to send the troops earlier on Monday.
The New Komeito party had expressed reservations about the deployment.
It commits Japanese troops to a combat zone for the first time since World War II.
Japan's constitution prohibits the use of force in international disputes, but the Japanese Government argues the country is entitled to exercise self-defence.
PACIFISM UNDER THREAT?
Japan's constitution renounces the use of force
This has been stretched to allow self-defence troops
1992 law allowed troops to join UN and relief work overseas
2003 law said troops could go to non-combat zones in Iraq
PM Koizumi wants to give Japan even greater powers
The troops in Iraq will mainly be supplying water and medical supplies, but will be able to return fire if they are attacked.
Public concerns that they could be targeted will be deepened by news on Monday of an attack on a lorry carrying equipment for Japanese troops near Baghdad.
The lorry carrying a mobile home for Japanese troops was attacked on the road between Jordan and Baghdad, killing the Jordanian driver.
No other details are available, but a spokesman for the Japanese defence agency said it was not judged to have been an attack specifically targeting Japan as the lorry was unmarked.
A total of about 1,000 Japanese troops are expected in Iraq by March.
Several thousand opponents staged a protest rally in Tokyo on Sunday to oppose the troop deployment. Opinion polls suggest that about half of the Japanese public is currently opposed to the deployment.
Mr Koizumi reiterated during a parliamentary debate on Monday that the troop deployment was needed as part of Japan's role in the international community.
"As Iraq's reconstruction is vital for the world's peace and stability, Japan should think about giving as much assistance as possible that matches the country's power," he said.