Japanese Premier Takeo Fukuda and other ministers who were once seriously troubled by the coming withdrawal of American ground forces from South Korea raised no objections today when U.S. Defense Secretary Harold Brown explained the five-year plan.

Before leaving for the United States tonight, Brown briefed Fukuda and two other Cabinet ministers on the withdrawal agreement he had reached in talks with South Korea leaders during the preceding two days. He emphasied the determination of the United States to remain a major power in the Western Pacific and East Asia region. The American role is crucial to Japan's security and Brown apparently satisfied the three ministers on the point.

Asao Mihara, director general of the Defense Agency, told Brown he welcomed the agreement. Japanese sources quoted Foreign Minister Iichiro Hatoyama as telling Brown: "We don't have any disagreement at all."

The meetings were remarkable for the apparent lack of concern on the part of the Japanese. After President Carter's election victory in November, Japanese officials outspokenly opposed the withdrawal plan, saying it might destablize the Korean peninsula and endanger Japan.

PrivatelY, Japanese officials now say their early anxiety was caused by a sloppy presentation of Carter's proposals.

"The first reports were bad and confusing," said a Defense Agency source. "They caused unnecessary doubt and suspicion among Koreans and Japanese."

Washington's subsequent cautions management of the withdrawal project has largely allayed those fears.

"We became convinced the United States is not abandoning Japan," said the source. "I imagine the Japanese leaders were genuinely convinced the pullout from Korea is just a restructuring of forces and not a change of commitment in Northeast Asia."

The questions asked of Brown today points to areas of continuing Japanese concern. Brown was asked whether the withdrawal would stop if Congress should refuse to authorize funds to improve South Korea's military strength. The defense secretary said he believes Congress will support the Carter program.

Many Japanese government officials think the United States made a mistake in announcing the withdrawal without securing a commitment from China and the Soivet Union not to help North Korea exploit the pullout. Today, Brown said he believes neither of the major Communist powers wants to see renewed conflict in Korea. Japan Defense Agency officials share that opinion.

The Japanese are acutely sensitive to changes on the Korean peninsula because a major conflict there would disrupt Japan. A flood of refugees, political polarization, and open strife between pro-North and pro-South elements among Japan's 600,000 Korean residents could follow. Beyond that, the Japanese fear that if Korea is reunified its ancient enmity toward Japan may be revised. The present divided situation on the Korean peninsula best suits Japan's national interests, and the prospect that the U.S. withdrawal would create a dangerous imbalance triggered alarm.

Now that the South Koreans have been convinced that the United States remains committed to their defense, the Japanese are ready to get in line. They have always seen less likelihood of a second invasion by North Korea than the government of Seoul.

When the troop plan was announced, the Japanese feared a complete and irresponsible withdrawal. Japanese security experts, who regard the Soviet Union as the major threat, began to doubt that the United States was willing to maintain a balancing presence in Asia. A U.S. pullback would leave the expanding Soviet navy free to use its new Pacific bases on the Kamchatka peninsula to dominate Japan's air and sea routes.

Speculation along these lines was rife when Vice President Walter Mondale visited Japan in February. His explanation of U.S. intentions was reassuring, and the prime minister's office passed the word to other government agencies that noisy opposition to the withdrawal would be unwise.

The Japanese were further encouraged by the restrained response of the United States and North Korea to the shooting down of a U.S. Army helicopter in North Korean July 14.

After the withdrawal agreement was announced in Seoul yesterday, the Japanese expressed satisfaction particularly at the firmly stated American commitment to stability in northeast Asia and the plan to station U.S. air forces in South Korea indefinitely.

One defense agency official criticized the U.S. promise to help South Korea build up its defense industries. He said this might lead to an arms race between North and South Korea that could eventually involve Japan.