Foreign Ministry spokesman Hatsuhisa Takashima said on Wednesday that all permanent members of the UN Security Council should be treated the same as other members.
"Japan considers it unfavourable to have permanent Security Council members that have veto power and those that don't have it," Takashima said.
But the spokesman added that Japan would bow to the majority within the UN "if it is the consensus of the international community".
Reform under way
A high-level panel on reforming the UN appointed by Secretary-General Kofi Annan has proposed two models for reform, including one that would add six new permanent members to the council but without the power of veto.
Japan, along with Germany, has long sought a permanent seat on the Security Council. Annan wants a decision on reform next year.
"Japan considers it unfavourable to have permanent Security Council members that have veto power and those that don't have it"
foreign ministry spokesman
Any change to the council membership needs approval from two-thirds of the 191-member General Assembly and must avoid a veto by any of the council's current permanent members.
Britain, France and Russia have indicated their support for Germany, Brazil, India and Japan. China, parts of which were under Japanese rule until 1945, has difficult diplomatic relations with Japan and is thought unlikely to support its bid.
The US has expressed support for Japan but the Bush administration has not offered similar backing to Germany, which opposed the US-led Iraq war.
Japan's Takashima welcomed the panel's recommendation that the "enemy state" clause be removed from the UN Charter.
The clause, dating back to the second world war, allows for military action against Japan and Germany, without any endorsement by the Security Council.
Japan contributes almost as much money as the US to the United Nations.