The Sultan of Brunei, arguably the richest man in the world, is reclusive, imperious and unlikely to be toppled from power in the small Asian country he has ruled for 20 years.

Americans know him only as a royal footnote in the Iran-contra scandal -- the man who was so wealthy he didn't bother to inquire about a missing $10 million he had contributed to the contras, which wound up in the wrong Swiss bank account because of a typing error.

But behind the scenes in the investment world, Sultan Muda Hassanal Bolkiah has become a major force. It is believed, for instance, that Prime Minister Margaret Thatcher asked him to transfer some of his funds to British banks in early 1985 -- which he did, thus singlehandedly propping up the British pound.

Who is this multibillionaire? Dale Van Atta traveled to the tiny, oil-rich Moslem sultanate on the north coast of the island of Borneo to find out. The press-shy potentate turned down an interview request, pleading ill health. But associates and some of his subjects were quite willing to talk.

By all accounts, the amount of the sultan's wealth is staggering. For instance, the sultan personally controls investments made from Brunei's foreign reserves -- nearly $20 billion. The amount is equivalent to two-thirds of America's foreign reserves, and is more than the foreign reserves of either Great Britain or Switzerland.

The 41-year-old ruler does not hide his wealth, either; he flaunts it. He lives in the largest house in the world, a palace with 1,788 rooms, which was completed just in time for Brunei's independence day on Jan. 1, 1984. Though many of those rooms are used for government personnel, some 944 are for the "private" use of the sultan, his two wives, seven children, their relatives and friends.

The sultan's spending habits are not criticized in the government-owned press. Privately, however, his subjects can be quite acerbic. One observed -- with more truth than wit -- that "the only thing the sultan cannot afford is democracy."

The sultan is the 29th ruler in an unbroken royal line. His father took a stab at democracy in 1962 -- recanting quickly when an opposition party won 35 of 36 legislative seats. The subsequent mildly leftist rebellion, in which 2,000 died, was put down by British Gurkha units flown in from Singapore. The present sultan rents these Gurkhas from Britain as a deterrent to would-be rebels.

Even more successful in keeping the peace is the sultan's dole system. Bruneians have one of the highest per capita incomes in the world -- some $20,000 a year. Half of the work force is employed by the government. Government subsidies and no-interest or low-interest loans also benefit Bruneians in housing construction, food and auto purchases, and medical and educational services. There is no personal income tax.

All this spells peace among the sultan's 232,000 subjects for the near future. But serious problems are around the corner. Brunei has few farmers: Four-fifths of its food is imported. In fact, its beef is raised on a ranch Brunei owns in Australia which is bigger than Brunei itself. Should the country ever be cut off from the outside world, these wealthy people might well starve.