A top Soviet economist said his nation is moving toward a "step-by-step" approach that could one day result in a ruble that would be freely traded on world markets.

Valeriy Ribin, chief researcher in economics at the prestigious Academy of Sciences in Moscow, said a convertible ruble is among a "great variety" of changes being considered by Soviet economists.Other changes under discussion include a Soviet stock market as well as the lifting of government restrictions on the ruble, the Soviet Union's currency, Ribin said.

He confirmed that Soviet economists are "preliminarily discussing . . . the creation of the first Soviet stock market, which would have a chance of direct contact with worldwide stock exchange markets."

But he emphasized that the buying and selling of securities must not be allowed to take precedence over other urgent and economic and political needs, such as consumer demand for better food, clothing and retail goods.

"We think that we must create a stock market in our country, but it should not outpace the . . . creation of the goods market," Ribin said.

Ribin, part of a delegation of five Soviet economists on a two-week trip to the United States, spoke at a seminar at the Federal Reserve Bank of San Francisco.

Speaking through an interpreter, Ribin said the Soviet Union "is now taking measures" towards the eventual convertibility of the ruble as part of a campaign of economic reform.

Currently, rubles are redeemable only by the Soviet government and are not traded on world currency exchanges.

Government economists are contemplating a "step-by-step" approach in which the ruble, now worth about $1.60, could be made convertible in certain markets or particular regions to be designated by the state.

However, the Soviet Union likely will test market a convertible ruble in Eastern Europe before it allows it to be traded on world markets.

"I can predict the introduction of the convertibility of the Soviet ruble in our relations with other communist countries," Ribin said, but he added, "it would take more time to introduce the convertible ruble (to) the world market."

Making the ruble convertible would be a boon to U.S. and other foreign companies that hope to do business in the Soviet Union and would like to convert their profits into currencies other than the ruble.

A convertible ruble would also help Soviet companies become more competitive in international trade.