The Japanese government is considering a multibillion-dollar energy conversion program that, if carried out, would dramatically reduce this country's reliance on oil by the end of the 1980s.

If it is successful, it would mean that by 1990 only about 48 percent of Japan's energy would be generated from oil, as compared to the current 74 percent, and that total oil imports would be far less than now anticipated.

The plan calls for a massive shift to other energy sources - solar, nuclear, imported coal and synthetic fuels.

To pay for the conversion plan, the government is considering imposing a substantial tax on consumption of both oil products and electricity. The total cost over the next 11 years could run as high as 5 trillion yen, about $23 billion.

It is by far the most ambitious energy plan set forth for Japan, which is heavily dependent on foreign oil for energy. Its emergence at this time reflects far more concern and impatience than has been publicly expressed since the current oil shortage began this year.

The program was made public by Masumi Esaki, head of the powerful Ministry of International Trade and Industry. He has emerged in recent weeks as a forceful spokesman both for shifting away from oil and for ensuring current oil supplies from sources in the Middle East.

The Japanese Cabinet studied the plan today and, although he gave no precise endorsement, Finance Minister Ippei Kaneko indicated he would consider the specific proposals in formulating the government's new tight budget. The Trade Ministry is expected to have prepared legislation by the end of August.

Japan is potentially one of the world's most vulnerable countries in times of oil shortages because it imports more than 95 percent of the oil used in industries and homes.

Unlike the 1973 oil shock, which spread panic through Japan, this year's has been greeted relatively calmly. The government has urged conservation and taken a few minor actions to reduce air conditioning and weekend driving.

Prime Minister Masayoshi Ohira had pledged earlier to "spare no effort to solve the problem" but had indicated nothing so drastic as the 11-year plan unfolded by his Cabinet minister, Esaki.

The goal, Esaki told the press, is to limit Japan's importation of foreign oil to 6.3 million barrels a day by 1990. A recent estimate projected imports at 7.8 million barrels a day by that time. Japan currently imports about 5.4 million barrels daily, most of it from the Middle East.

He said Japan could fill part of that gap by using imported coal and estimated it would require bringing in about 100 million tons a year by 1990, most of it from China and Australia.

It would also mean promoting nuclear power, an increased use of liquified natural gas, and a far greater reliance on solar energy, he said.

To some observers in the government, the Esaki plan seems rather optimistic. For instance, Japan currently generates very little energy from the sun's rays, but Esaki said that by 1990 one out of every five households should be relying on solar power. Esaki proposed government subsidies to assist 8 million homeowners in switching to solar heating and lighting.

In addition, the buildup of nuclear power has slowed in Japan under the same type of environmentalist pressures that operate in the United States and the government has been especially cautious in promoting it - even before the safety issue was raised alarmingly by the Three Mile Island accident in Pennsylvania.

According to Esaki, most of the money to promote the changeover to other energy sources would be raised by levying a 2 or 3 percent tax on consumption of oil and electricity. Those taxes would pay for about three-fifths of the program, and the rest would have to come from government loans and investment programs.

Despite the measure of approval offered by the finance minister today the tax program would probably encounter stiff resistance from the governing Liberal Democratic Party. A parliamentary election is expected this fall and the government already is pondering a new excise tax to make up for current deficits.

Japan also has launched a diplomatic campaign since the Tokyo summit to retain and expand its foreign sources of oil.

In an unusual Middle East tour, Esaki visited Iraq, Kuwait, Saudi Arabia and the United Arab Emirates, all of which are principal oil suppliers. In addition, Japan has attempted to secure new supplies from Mexico and is negotiating seriously with China for joint development of oil reserves believed to lie beneath the Senkaku Islands, which both countries claim to own.