Chiba, Japan — ''The Japanese-American relationship is our fundamental foreign policy line, '' said Prime Minister Yasuhiro Nakasone. ''Whatever the results of the election, there will be no change in this line.''
The Japanese prime minister, now in his final campaign week before the general election Dec. 18, spoke to foreign journalists during a brief interval between speeches in this city adjoining Tokyo.
Mr. Nakasone made no specific promises regarding either defense or economic ties, the two areas of greatest concern to Washington. But in speeches from balmy Kagoshima to snowbound Hokkaido, he has contended that a convincing victory for his ruling Liberal Democratic Party will strengthen his hand in promoting Japanese-American cooperation and in making Japan a more responsible member of the world community.
Since becoming prime minister a year ago, Mr. Nakasone has tried to popularize a new goal for Japan: to become an ''international state'' (kokusai kokka).
What he means by this term, he told supporters in Chiba Dec. 13, is a country no longer thought of by others as selfish or unfair or closed to foreign goods, but one prepared to play a responsible role in world affairs and with a voice that is respected internationally.
Shigeru Yoshida, architect of Japan's postwar recovery, made peace and the economy Japan's twin goals, Mr. Nakasone said from atop his campaign van, waving his white-gloved hand.
''Thanks to the efforts of you, the people, we have become the second largest economy in the world. But there has been a tendency during these years for other countries to think of Japan as a country that thinks only of itself, as being unfair and closed to goods from outside. Thus Japan began to be isolated from other countries. This is very dangerous.''
He then quoted the late futurologist Herman Kahn's prediction that ''the 21st century will be Japan's century,'' but on two conditions: that the Japanese people do not forget the need for international cooperation, and that they do not lose the habit of hard work.
''Japan lives by trade,'' Mr. Nakasone said. ''We must handle international cooperation well. We must increase our friends throughout the world. We have got to move on from a stage in which we are thought of just as an economically shrewd people to one in which our voice is heard with respect around the world.''
''To be labeled unfair,'' Mr. Nakasone said in a subsequent speech back in Tokyo, ''is to be considered a trickster. How humiliating. We are a country with 2,000 years of culture and tradition. We must sweep away the misunderstandings of others, and if in any respect we should change our way of doing things, we should do so.''
Foreign policy is Mr. Nakasone's strong suit. More than any previous Japanese prime minister, he has emphasized the importance of close Japanese-American ties , both in defense and in trade. He is the first Japanese prime minister to establish a ''Ron-Yasu'' first-name relationship with an American president. He has spoken out strongly and clearly against the danger of Soviet expansionism and rejected the opposition Socialists call for ''unarmed neutrality'' as a ''dangerous invitation to would-be aggressors.'' He has cultivated friendship with South Korea, with China, and with the Association of Southeast Asian Nations (ASEAN).
Public opinion polls show that voters are much more pleased with Nakasone's record in foreign policy than they are with his domestic record. In a poll conducted by the Yomiuri newspaper last month, 58 percent of the respondents applauded his foreign policy, including his strengthening of Japanese-American ties. Less than a third liked his record in stimulating the economy or in reducing taxes.
As for political morality, only 2.4 percent said they gave a ''high evaluation'' to the Nakasone Cabinet's record; 20.9 percent said they could give ''some credit'' to the Cabinet in this field; while 36.1 percent said they could give ''little credit'' and 20.7 percent said they could give ''no credit at all.''
Thus the political morality issue, focussed on former prime minister Kakuei Tanaka's refusal to resign his seat in the Diet after being convicted of taking a bribe, remains the sleeper in this campaign.
Few voters seem to be thinking of not giving the Liberal-Democrats a victory, as they have in every election since 1955. But if enough voters give grudging rather than convincing support to the Liberal-Democrats, the party's majority in the Diet could be drastically reduced. And that, in turn, is bound to weaken Mr. Nakasone's hand in foreign affairs, including relations with the United States.