Japan's Sony Corporation bought Columbia Pictures last month. Now Mitsubishi is paying $846 million for 51 percent of the Rockefeller Group, owner of Rockefeller Center, the vibrant Art Deco cluster of 19 buildings in the heart of New York City. Is the transfer of American assets to Japanese ownership something to worry about?

In an immediate and practical sense, the answer is no. Foreign investment is a mark of confidence in the U.S. economy. British and Dutch investors have accumulated large American holdings without causing great anxiety. It would be xenophobic, even racist, to single out Japan because of the size of its American holdings, which, though growing fast, are still half the size of Britain's.

It's also hard to object to specific sales like that of Rockefeller Center. Its owner is happy to have found a buyer. And the sale will bolster New York's commercial real estate market.

So why worry? Even if the concern is mainly over symbols, many people are puzzled that the centerpiece of America's most renowned metropolis is about to pass into foreign ownership. The leading American corporations that have offices in Rockefeller Center - NBC, Time Warner and Morgan Stanley - will now have a Japanese landlord. What's happened? Japan's trade surpluses with the United States have persisted at about $50 billion a year. These dollars eventually come back to America, and one form in which they have done so has been a wave of Japanese investment in real estate. Japanese companies already own 30 percent of downtown Los Angeles and much of Honolulu.