Japan's ex-ambassador to Lebanon, Naoto Amaki, said on Monday about 50 lawyers from the Hokaido area believed Junichiro Koizumi had violated the country's pacifist constitution.
Amaki, who was forced to resign his ambassadorial role because of his opposition to the Iraq war, said the case was part of a nationwide campaign to force Koizumi to change his mind.
The news comes as Japan's defence agency formally ordered the dispatch of 600 troops to Iraq.
The first soldiers would leave on 3 February, joining a handful of advance troops already in Samawa in southern Iraq.
The defence agency's approval was issued after Koizumi won the backing of his junior coalition partner.
Pressure on PM
"It is not easy to make the PM resign but even if we don't succeed in this the impact of this case will be huge. No one has ever sued the PM before"
Ex Japanese ambassador to Lebanon
However, Amaki said he hoped the pressure on the prime minister would force him to step down.
"It is not easy to make the PM resign but even if we don't succeed in this the impact of this case will be huge. No one has ever sued the PM before.
"And in a sense it doesn't matter what the court says because we all know that Japanese courts avoid judgement on political matters."
He added: "The US will for the first time in postwar history realise that it has to face the will of the Japanese people even if it takes for granted that it can control the Liberal Democratic Party leader (Koizumi)."
The government's decision to send troops to Iraq to assist the US occupation is hugely controversial in Japan.
A recent opinion poll revealed the country is equally split between those who support and those who oppose the move.
Koizumi has commited Japanese troops to a combat zone for the first time since World War II.
Many believe this violates the Japanese constitution which prohibits the use of force in international disputes.
However, the Japanese government argues the country is entitled to exercise self-defence.
Japanese troops have not seen
combat since World War II
And Koizumi says Iraq's reconstruction is vital for world peace and stability and Japan should give as much assistance as possible.
Many Japanese also believe thier country's economic and political fate is closely tied to the US, and American protection is needed to counter the potential threat from North Korea.
Japanese troops in Iraq will be supplying water and medical supplies mainly, but will be able to return fire if they are attacked.
About 1000 Japanese troops are expected in Iraq by March.