The United States, in unusually strong criticism of its senior ally in Asia, yesterday called Japan's new military budget disappointing, inadequte and tinged with complacency that is unjustified in a troubled world.

Beyond the carefully crafted but stiff public pronouncements, knowledgeable officials expressed private dismay at the budgetary news from Tokyo. The officials said the Japanese decision is likely to set the stage for serious conflict with the incoming Reagan administration and with elements of Congress and the public that have charged Japan with taking a "free ride" in the military sphere.

Secretary of Defense Harold Brown, who has strongly urged a substantial Japanese military increase, said in a statement that the proposed 7.6 percent budgetary increase "is so modest that it conveys a sense of complacency that is not justified by the facts."

Brown said the increase "falls seriously short" by a number of measurements, including that of "equitable burden sharing" among free nations.

The State Department, in a separate statement, said "the Japanese decision must be considered disappointing, whether one measures these defense spending figures against the target set by Japanese defense officials earlier this year, or against the requirements of equity in distributing the burdens of mutual security among the advanced industrial democracies."

Although Japan's defense decision had been hotly and openly contested in Tokyo for several months, Washington officials said there was little warning that the budgetary increases would be as small as they were, well below sums that had been expected. One closely informed State Department official, for example, said he had been "dumbfounded" at the official news from Tokyo.

The projected budgetary increase of 7.6 percent includes about 2.2 percent in pay raises that had been expected to be considered separately, U.S. officials said. The resulting non-personnel rise of about 5.4 percent is likely t be less than 4 percent when adjusted for inflation, according to preliminary U.S. estimates.

The State Department statement said that, when adjusted for inflation, the Japanese military increase will be "significantly less than our own despite the much smaller base of Japanese defense spending."

In a discussion that began on a Brown trip to Asia last January, shortly after the Soviet invasion of Afghanistan, the United States publicly and privately urged a "steady and significant increase" in Japanese military spending, currently among the lowest of any leading industrial nation.

Following further discussions last spring, the Japanese Ministry of Finance established a tentative military budget target of a 9.7 percent increase to the Japan Defense Agency. U.S. officials said they were led to believe that rising personnel costs would be tacked on to this, making the actual defense boost well over 10 percent in nominal terms.

These figures were less than Washington thought was needed under the circimstances, but were accepted in public and private talk as the minimum necessary increase, according to the U.S. sources. Thus the Carter administration was dismayed, as well as surprised,to find the acutal Japanese performance well below these estimates.

The core of the U.S. argument is that Japan, with the world's third-largest economy, can well afford more than its very modest post-World War II defense effort. Japanese officials have argued that their country must proceed cautiously because of the anti-military feeling and constitution that followed defeat in World War II, and because of budgetary pressures at home.