America exports to Japan increased modestly in the third quarter, while imports from Janpan grew less than one per cent, resulting in the first quarterly improvement in the U.S. bi-lateral trade balance in two years.

But these figures, released by the Department of Commerce, still showed a trade deficit of $2.98 billion with Japan, down $263 million on a seasonally adjusted basis from the second quarter.

Officials expect that the trade deficit for the year as a whole will exceed $13 billion, compared with about $8 billion in 1977.

Because the figures reflect only one quarter's improvement, officials deliberately played the announcement low key. But Assistant Secretary Frank Well said in an interview: "I think we have seen the tide change."

Government officials are anxious to see a significent and continuing downward trend in the deficit with Japan, an important component of the overall red ink that has been putting pressure on the dollar.

Secretary of Commerce Juanita Kreps recently led a mission of American businessmen to Japan, seeking to encourage a greater American effort to export goods to Japan, and at the same time urging the Japanese to accelerate efforts dismantle protectionist barriers.

Privately, U.S. officials in many agencies concede that unless there is a dramatic turn-around in the deficit numbers by next Spring, there will be growing political pressure to put an import surcharge on Japanese exports to this country.

Yesterday's figures were viewed by trade experts at least mildly encouraging. The September U.S. trade deficit with Japan was put at $901 million, seasonally adjusted, the smallest monthly trade deficit of the year. It was also the second consecutive month of improvement.

On the import side, the growth of less than 1 percent during the quarter compared with a growth of 12 percent in the first quarter and 8 percent in the second quarter.

The Department said that the rate of growth of imports of cars and other automotive products, "while still substantial," had slowed in recent months. But there has been no change in the high level of imports of consumer electronics items and of steel.

The actual data, The Washington Post learned, showed that Japanese automotive exports to the U.S. in the third quarter 1978 were up 44 percent over the same quarter of 1977, whereas the gain in the second quarter had been 67 percent.

Growth in U.S. exports to Japan, according to the department, was "widespread" in the third quarter, amounting to a 10 percent gain, seasonally adjusted. The second quarter had shown a 15 percent gain, but over a first quarter which had actually declined 2.6 percent.

Fastest growing U.S. export categories in the third quarter were capital and consumer goods Increases were also shown in industrial materials, foods, feeds, and beverage exports.