TOKYO, June 4 — Japan ratified an international accord on limiting emissions of heat-trapping gasses today, ending more than six months of internal debate, and said it would lobby the United States and other large polluters to do the same.

Japan's ratification of the agreement, despite the opposition of many major industries, was an important step forward for the accord, known as the Kyoto Protocol on global warming.

"In order to ensure the effectiveness of measures against global warming, it is essential that all countries make efforts to reduce greenhouse gas emissions," Prime Minister Junichiro Koizumi said in a statement.

Japan is the fourth-largest emitter of carbon dioxide, which is produced by the burning of fossil fuels and is believed to be the most important heat-trapping gas and the main cause of global warming. The leading polluters are the United States, the European Union and Russia.

To meet the politically complex conditions for the Kyoto accord to become international law, it must be ratified by at least 55 countries and by the industrialized nations that emitted at least 55 percent of the industrialized world's carbon dioxide in 1990. Despite the rejection of Kyoto by the United States, the protocol could take effect with ratifications by the European Union, Russia, Japan and either Canada or Poland.

The 15 countries of the European Union, acting in unison, presented their ratifications of the protocol on Friday, but the United States and Russia have not agreed to its terms. The Bush administration has rejected the agreement on economic and scientific grounds, saying that emissions reductions would be costly, and asserting that the causes of global warming are poorly understood.

Recently, however, the United States has acknowledged a link between human activity and global warming. Russia, whose cabinet has given preliminary approval to the accord, is holding out for extra credits toward emissions targets and may not ratify it until next year, say people who are following the issue.

Under the protocol, industrialized nations would be required to cut emissions of greenhouse gases by an average of 5 percent over the period 2008 through 2012, compared with 1990 levels. Japan has pledged to cut its emissions by 6 percent over this period.

Environmentalists point to a substantial rise in Japan's output of greenhouse gasses since 1990 and say the country will have difficulty meeting its target. "Japan is going to be very behind in its efforts to achieve its target without new measures," said Yurika Ayukawa, of World Wide Fund for Nature Japan, according to Reuters news service.

Japan's ratification of the protocol appeared in doubt late last year, with the powerful Finance and Foreign Ministries reportedly joining to oppose it. The Environment Ministry was the accord's strongest advocate, aided by the fact that the protocol is named after the Japanese city where it was signed during a United Nations conference.

When a former environment minister who was involved in negotiating the pact, Yoriko Kawaguchi, took over as foreign minister recently, she put her office's weight behind the emissions control plan. "I am deeply moved and pleased that Japan will become a party to the Kyoto protocol," Ms. Kawaguchi said today.