When a little-known politician named Zenko Suzuki became prime minister last summer the wise heads in Tokyo described him as an amiable fellow with one major fault -- he didn't know much about foreign policy.

The judgement seemed to be proved accurate today with the sudden resignation of his two top foreign policy officials in a dispute that hinged on what Suzuki had or had not promised President Reagan last week.

Suzuki emerged from the dispute looking inconsistent and confused. His ruling faction in lthe Liberal Democratic Party was split, rumors of more resignations surfaced and the future of his goverment seemed uncertain.

Foreign Minister Masayoshi Ito resigned unexpectedly after a bruising intra-Cabinet squabble, announcing that he would take repsonsibility for the confusion that erupted over a joint communique issued by Suzuki and Reagan a week ago in Washington. The Ministry's top career offical, Vice Minister Masuo Takashima, also resigned and there were reports that lesser officials would follow suit.

Ito was replaced by Sunao Sonodo, 67, a former foreign minister who the 1978 Japan-China peace and friendship treaty. Sonoda, health and welfare minister since last September, was foreign minister from November 1977 to November 1979.

It was the unlikely culmination of a simmering disagreement over what had seemed to be merely a semantic confusion but which had touched on the raw nerves of Japanese politicians who don't want this country to play a bigger defense role in the world.

The communique last week had described the U.S.-Japanese relationship as an "alliance," which in Japan is a risky code word usually taken to mana a joint military undertaking. For a week, opposition parties and part of the press pilloried Suzuki for allegedly bowing to U.S. demand for an expanded defense effort with the United States.

[A State Department spokeswoman said the United States views the resignation as "an internal matter" and added that "there is no disagreement between the United States and Japan over the joint communique." Asked for the U.S. interpretation of the word "alliance" in that document, she said, "We believe this expresses the shared values that the United States enjoys with Japan." She declined to elaborate.]

Suzuki, throughout the week, denied that the term had any military connotation and stressed repeatedly that his talks with Reagan had not addded anything new to his country's military promises.

The prime minister finally triggered Ito's resignation by blaming the Foreign Ministry for mishandling the joint communique. Publicly in parliament and privately in an acrimonious Cabinet meeting, he said the communique did not represent his true views of the summit meeting.

He said in his final meeting with Reagan he had stressed that Japan's defense posture is sharply limited by public opinion, which opposes any big buildup, and by fiscal restratints imposed by heavy deficit spending.

Suzuki, it was learned, repeated these complaints to Ito's face during a heated Cabinet meeting yesterday. Sources said Ito replied by asking that the comments not be made public and reportedly believed he had received assurances of that. Instead, the confrontation was widely reported in newspapers, and at 11 p.m. last night Ito handed in his resignation. He was asked by chief Cabinet Secretary Kiichi Miyazawa to reconsider, but refused.

That produced a bitter disagreement within the Foreign Ministry, which felt it was being made the scapegoat for what was, in fact, Suzuki's own turnabout under political pressure. Takashima resigned to support ready to do likewise.

At no point in the controversy did Suzuki explain why he authorized the joint communique if he did not agree with the wording.

It was the first major disruption in the Suzuki governemnt since he became prime minister last July, following the death of Masayoshi Ohira, who suffered a heart attack.

There had been one sharp difference earlier this year, also involving an American issue. Suzuki authorized the Ministry of International Trade and Industry to negotiate the issue of restricting automible exports to the United States as Reagan requested. Ito had wanted his Foreign Ministry to negotiate the export problem.

The long-term political damage to Suzuki was not clear tonight. Since he and Ito are members of the same dominant faction within the Liberal Democratic Party, the incident was considered to cause a factional split. There was specualtion that opposing party leaders might try to take advantage of the dispute to bring Suzuki down. Even before the Ito resigantion today, the prime minister was being criticized in the press for inconsistency.

One result of the affair is that Suzuki has retreated far from the impression he left in Washington about Japan's willingness to contribute more to Pacific defense. Although he had said nothing substantively new, the tone of his comments had left the impression he was pointing Japan in some new directions, including a "division of roles" with U.S. forces in the Pacific.

Since returning to Tokyo, however, Suzuki had painstakingly refuted any suggestions that he offered Reagan an expanded defense role. Japan will not consider any form of collective defense with the United States and had not promised to increase defense expenditures, he told parliament yesterday.