Japan and the United States have expressed a willingness to join in a feasibility study for a second Panama Canal, a project energetically promoted in recent months by Panamanian officals and influential Japanese businessmen.

The second canal, which might cost $20 billion, is being pictured by business interests here as a revolutionary development in world trade worth hundreds of million of dollars a year to Japan.

Their campaign to obtain official approval and financial support began to pay off this week as Prime Minister Massayoshi Ohira indicated that Japan would participate in an expensive feasibility study if the United States would also take part.

American sources said the Carter administration endorsed the study during Foreign Minister Saburo Okita's visit to Washington last week.

Although committed by an agreement with Panama to make the study someday, the United States had shown little interest in getting it under way until Okita's visit. An embassy spokesman confirmed that American officials gave it a "green light" last week and said: "The Carter administration is favorably disposed to U.S. participation in a sea-level canal feasibility study. We are sympathetic and look forward to the study."

Panamanian President Aristides Royo, who is here on a state visit, discussed the canal study with Ohira this week and in a joint communique tonight expressed hope that discussions involving the United States would be held soon. It quoted Ohira as saying Japan is ready to take a "positive initiative" for a multifaceted study of the canal.

Royo said later at a news conference the matter had been discussed with Carter. He also emphasized that the canal's "neutrailty is essential."

Proponents of a second canal contend that it will be necessary to supplement the present one, which is too small to handle, the larger modern vessels now carryng most of the world's trade, particularly in oil and raw materials.

Japan's most ardent proponent is Shigeo Nagano, honorary chairman of Japan Steel Corp. and president of the Japan Chamber of Commerce and Industry. One of Japan's most influential businessmen, he has worked closely with Royo to involve both the Japanese and American governments in the project.

In an interview this week, Nagano unfolded a preliminary plan, complete with map, of where the new canal should be located. It would run about 10 miles west of the present canal, from Puerto Caimito on the Pacific coast to Lagarto on the Atlantic coast. It would be more than 100 feet deep and at least 650 feet wide.

Operated without locks, the new canal could handle most large ocean-going vessels, including the large container ships and tankers that Japan depends on for raw materials and oil. Because Japan needs imports of both to survive, the accessibility of a quick route from the Atlantic nations and the Middle East would have great importance.

"I am a businessman, and we businessmen never speak of things that do not have benefits," Nigano said before outlining a second canal's advantages. He will be 80 years old in July and enxisions building the second canal as a project to occupy his interest until he is 90.

He said Japan could import soy beans, wheat and West Virginia's coal from the United States much less expensively through a second canal than via the present route around South America. It would also help Japan tap sources of oil in Venezuela and iron ore in other South American countries, he said.

In addition, Japan is worried about protecting its present main oil routes through Southeast Asia and would regard a Panama route as much safer.

He estimates that the combination of larger ships using a shorter route could mean "several hundreds of millions of dollars a year of benefits to Japan."

Nagano said that at normal times the new canal would accommodate ships of 300,000 tons and at high tides ships of 500,000 tons. The present canal, built in 1914, can handle ships up to 65,000 tons, he said. Ships of that size are carrying a steadily diminishing portion of world sea trade.

The business community here has argued that home-grown technology would be important because it was developed in the project to widen and deepen the Suez Canal. A Japanese company, Penta Ocean Construction Co., had a nearly two-thirds interst in that project, along with firms from France, Italy and the Netherlands.

Penta Ocean already has done a small study of a second canal for Nagano.The chairman and former president of that company is his younger brother, Toshio Nagano.

It estimates that 10 years would be required to build the canal. Construction would cost about $8.3 billion, and the entire project, including costs of infrastructure, would come to nearly $20 billion.

A second Panama Canal might have a new military importance from the U.S. standpoint, since it would probably be able to accommodate the Navy's modern supercarriers. Japan claims this would be significant in light of the new so-called "swing strategy" that calls for rapid shifting of U.S. naval forces between the Atlantic and Pacific oceans in times of crisis.

Nagano has argued his point in Washington with Commerce Department officials and at a luncheon with Senate Majority Leader Robert Byrd (D-W.Va.). He has worked closely with Sen. Mike Gravel (D-Alaska), whose interest is said to lie in the movement of Alaskan oil to eastern U.S. refineries.