For Mstislav Rostropovich, music director of the National Symphony Orchestra, it was something of a homecoming in Japan, and he wasn't about to let a little thing like small audiences dim his enthusiasm.

Japanese audiences, he proclaimed, are "subarashii," the Japanese word for "wonderful." He has, he said, "a very deep love for this country." Then he polished off a short speech to the Japanese press with a creditable "domo arigato gozaimasu," which means thanks a whole lot.

The effusive conductor led the symphony in a Tokyo abloom with cherry blossoms today after the first performances on the orchestra's five-nation Asian tour got off to a rather inauspicious start.

At the first stop, in Osaka last week, two concerts in that city's festival hall drew far less than capacity crowds and an even smaller hall in Nagoya was not quite full.

Only one of the symphony's three Tokyo area performances will be played in a major concert hall, the Tokyo Bunka Kaikan. The others are in small college auditoriums.

It was the Osaka appearance that provided Rostropovich with a taste of homecoming. Twenty-five years ago he played in the first Osaka music festival with the Leningrad Symphony. "That was my first trip here," he said today, "and it makes me feel very young to be back again."

The symphony caravan numbers 178 persons, including families of some members, staff, stage hands and a doctor. While in Japan, they are being treated to some displays of this country's technological achievements. In Osaka, they saw an entire chorus lifted from an orchestra pit on a mammoth hydraulic lift while the Japanese national anthem was sung. This afternoon it was the electric music of the Yamaha Company's youth performers, who pounded out deafening numbers on electric organs for Rostropovich's benefit.

After finishing the Japanese tour Thursday, the symphony heads for performances in the Philippines, Hong Kong, Taiwan and South Korea.

At a press conference that meandered in and out of Japanese, Russian (Rostropovich's native tongue) and English, the music director and his staff somehow got caught in a political question on why China was not included on the tour.

Henry Fogel, the orchestra's executive director, said there was "no political message" in not including mainland China on the schedule. It had nothing to do, everyone insisted, with the current furor over Hu Na, the Chinese tennis player whose defection to the United States has enraged the Communists and halted all cultural exchanges between the United States and China.

Rostropovich, himself a political exile from the Soviet Union, burst in to defend the decision to grant asylum to the Chinese athlete. "Who is supposed to send these people back if they don't want to go?" he asked.

Rostropovich, one of the world's leading cellists, has performed in Japan before, but on this trip has cut himself out of the act. "It's enough, just conducting," he said. "I can never conduct and play on the same day."

The orchestra has received little advance publicity for this tour, which may explain the small audiences outside Tokyo. Another reason may be economic. Japan is still feeling the worldwide recession and tickets for the symphony's performance are more than double the prices charged in Washington, which range from $4.50 to $19.50 a performance for non-season tickeholders.