The war-renouncing constitution - drafted by US occupation forces and unchanged since 1947 - bars the use of military force in settling international disputes and prohibits maintaining armed forces for warfare.
Tokya has interpreted that to mean the nation can maintain troops for self-defence and has one of the most modern, best-equipped militaries in the world.
However, Japanese troops have taken an increasingly high-profile role in recent years, prompting some critics to accuse Tokyo of moving away from its post-war pacifism.
Koizumi, who has strongly backed a bigger role for Japan's forces, on Wednesday said it was time for the troops to be openly referred to as a military.
"To still be saying that self-defence forces are in violation of the constitution strikes both the public and experts as strange," the Japanese premier said.
"Whether they're called self-defence forces or a self-defence military, an organisation to defend Japan should be clearly defined so that it does not raise issues of unconstitutionality."
A Self-Defence Force contingent
of 500 troops is deployed in Iraq
Earlier during parliamentary questioning, Koizumi agreed with opposition Democratic Party lawmaker Yukio Hatayama that the forces should "be clearly designated as a self-defence military" in the constitution.
Koizumi added that regardless of what the troops are called, "in the event they are dispatched abroad, they will not engage in the use of force".
Koizumi has prodded the country into open debate about constitutional reform, arguing that broad revisions are necessary for Japan's military to uphold its international responsibilities and for closer cooperation with Washington, which maintains 50,000 troops in Japan under a security treaty.
But Koizumi has also been criticised for taking steps that have stretched the limits of the constitution.
"From abroad, I
think it's the same whether you call it
a self-defence force or
a self-defence military"
Japanese Prime Minister
Koizumi has sent 500 troops to Iraq for humanitarian and reconstruction work, but many say they could get drawn into combat.
Earlier in 2001, he pushed through special legislation to let the navy provide logistical support to forces in Afghanistan for the US "war on terror".
Critics have said such efforts are chipping away at the pacifist society Japan has built since its destruction in World War II.
Asian neighbours, including South Korea and China, which suffered under Japan's militarist expansion before the war, have also watched the developments warily.
Koizumi dismissed that any change of name would raise alarm bells in the region.