Despite American appeals and its own growing apprehension about Soviet military moves, Japan shows no signs of increasing its defense budget as a response to the Soviet invasion of Afghanistan.

Japan will stick with a proposed budget that represents the lowest rate of increased military spending in recent years, according to reliable sources, and no plans have surfaced to suggest accelerated spending in the next few years.

These sources cite an extremely tight overall budget for the coming year which made it difficult even to reach the level of last year in terms of the amount spent on defense in relation to national income, the traditional measurement here.

Moreover, sources said that even though the Japanese military is increasingly worried about Soviet moves, there is no inclination on the part of political leaders to react by building a significantly larger defense base.

U.S. Secretary of Defense Harold Brown, in meetings here last month, urged Japan to undertake a steady and significant increase in defense spending, citing the Soviet invasion of Afghanistan as evidence of a need for noncommunist countries to strengthen defense.

He also left behind a hint that the United States would welcome an acceleration of Japan's defense spending up through the mid-1980s. It was the strongest message delivered here by an American official in recent years. American congressmen frequently call on Japan to spend more militarily, but the official U.S. position has been to advise only modest buildups of Japan's air and sea defenses.

The Japanese budget for 1980, prepared before Brown's visit, will not be changed and no supplemental spending will be proposed, government sources said this week.

For the past several years, defense spending has been increasing by about 8 to 10 percent a year, or at a real rate, taking inflation into account, of about 5 to 6 percent.

The new budget proposes a nominal growth of about 6.5 percent and a real growth of only about 2 percent with a high inflation rate predicted.

Traditionally, 1 percent of gross national product is the maximum defense allocation politically acceptable here, which is low by U.S. and Western European standards and far lower than neighboring South Korea's. The 1980 budget allocated about 0.9 percent of Japan's GNP last year.

An unusually tight government budget is the explanation offered by Japanese officials. Prime Minister Masayoshi Ohira promised this year to begin reducing Japan's dependence of borrowed funds and all agencies were ordered to submit bare-bones budgets. The Japan Defense Agency actually fared better than other departments.

Japan has denounced the Soviet invasion, but the Ohira government's public expressions have seemed designed to avoid any great sense of alarm. In a speech in late January, Ohira described the Soviet Union generally as a defensive-minded country which does not usually take unconsidered actions. In parliament, he referred to Soviet military might as a "potential threat" to Japan.

When the country's new defense chief, Kichizo Hosoda, called the Soviet Union a "serious threat" in his personal opinion, he was advised to retreat to Ohira's more cautious phrase.

Within the defense establishment, however, there are more signs of concern, especially with recent Soviet moves in areas near Japan. In the past two years, nearly 10,000 Soviet troops have been stationed on islands north of Japan and large task forces of Soviet ships have begun moving routinely through adjacent waters, headed for Southeast Asia or the Indian Ocean.

Last week, defense officials described for the first time new elements of the Soviet offensive buildup. The defense agency said about 10 medium-range SS20 ballistic missiles and about 10 Backfire bombers have been deployed recently in the Far East in range of Japan.

Because of these deployments and the Afghanistan affair, defense officials described the government as privately more concerned than it lets on publicly. But there is no consensus on what, if anything, Japan should do to counter them, they said.

Largely shaped by American requests, the Japanese defense plan calls for further increases in air and sea defenses which have been steadily built up in the past decade. The new budget for example, calls for a second contigent of 10 P3C Orion long-range antisubmarine patrol aircraft and 34 new F15 jet fighters, to go with 23 bought from the U.S. earlier.