The Reagan administration should not interpret the tough line the new French government is taking against terrorism at home as automatic support for military action against Libya, Edouard Balladur, the most powerful figure in Prime Minister Jacques Chirac's Cabinet, warned here last week.

Responding to a question about reports of possible U.S. action against Libya and probable European reaction, Balladur said, "We have started a vigorous internal antiterrorist program in France that will protect public order and security. However, this does not lead automatically to France supporting actions by another country in the name of antiterrorism."

Balladur, minister of state for finance, economy and denationalization as well as Chirac's chief strategist and political adviser, also indicated in a rare interview Friday that the conservative administration headed by Chirac has reached a clear consensus with Socialist President Francois Mitterrand on foreign policy questions and will seek to minimize any differences.

The net effect of his remarks was to suggest that while Chirac and his colleagues have effusively praised the Reagan administration's economic policies and promised to copy a number of them, the new government is unlikely to move toward significantly increased cooperation with Washington on international political matters.

On domestic economic questions, which dominated most of Friday's interview, Balladur expressed confidence that the conservative coalition that narrowly took control of the National Assembly from the Socialists March 16 would be able to implement its program of sharply reducing state control of the economy, despite some early obstacles Mitterrand has established.

Balladur's views on the ability of Mitterrand, whose seven-year presidential term does not expire until 1988, and Chirac, the leading conservative contender for the presidency, to avoid conflict carry particular authority. Working quietly behind the scenes, he was the principal architect of this unprecedented power-sharing arrangement, known in France as "cohabitation."

"It is the duty of the president and the prime minister to coexist," Balladur said. "So far it is working in a generally satisfactory manner."

As Chirac's eminence grise in recent years, Balladur shunned contact with the press and his own views have not been clearly known in France. But in a two-day visit to Washington this week to attend meetings of the International Monetary Fund, he indicated through the interview and a press conference for French reporters that the weight of his new role as de facto deputy prime minister will draw him out of the shadows.

Other points covered in the interview included:

*Balladur predicted that Mitterrand and Chirac would work smoothly together at the summit of seven industrial nations in Tokyo in early May. "There will be no problems of substance between them, and any protocol problems will be worked out" with ease, he said. Chirac will be the first French prime minister to insist on attending the economic gathering.

*Balladur portrayed the 6 percent devaluation of the French franc against the West German mark that he negotiated a week ago as a preemptive move that will give him more room to maneuver as he implements a program of lifting price controls, reducing interest rates, selling off nationalized firms to private investors and reducing public spending this year.

*Separate efforts for an international monetary conference undertaken in recent years by the United States and by France have lost most of their urgency, he indicated, as the dollar has fallen against other currencies. France would continue to express strong interest in achieving exchange rate stability, Balladur said, but his support for international controls and surveillance sounded far less enthusiastic than did that of his Socialist predecessors.

Given Chirac's deep involvement in and strong personal attraction to French electoral politics, Balladur is rapidly emerging as the man who will run the conservative administration on a day-to-day basis. He will be formally in charge of the Cabinet while Mitterrand and Chirac are in Tokyo.

Like Chirac, Balladur entered high-level French political life during the economically expansionary administration of the late president Georges Pompidou, whom Balladur served as chief of the staff at the Elysee in the early 1970s. Although Chirac and Balladur are called Gaullists, their political philosophies are closer to the probusiness views of Pompidou.

Vigorous at 57, Balladur is a man who carefully measures what he wants to say and then says it in clear and concise French without the ornate flourishes adopted by many of the political figures of his nation. He also avoids the economic jargon favored by some of his predecessors as finance minister, generally considered the most important job in the French Cabinet.

Described by one French associate as "courteous without being personally warm," Balladur has already earned marks as a skillful and determined negotiator in financial matters in his brief tenure. In a 24-hour tug of war last weekend, the Germans resisted Balladur's initial demand for an 8 percent devaluation of the franc against the mark, but wound up giving him most of what he wanted. One German official subsequently described Balladur to IMF officals here as "a strong man who knows what he wants."

Balladur said he had no apologies to offer as a Gaullist for proposing to denationalize firms that had been nationalized under de Gaulle or for moving away from interventionist policies often associated with de Gaulle's governments.

"You cannot run the economy of a country with 20-year-old methods," he said, arguing that de Gaulle was, in fact, a pragmatist. "The needs of the Second World War, of the reconstruction of France" required strong state intervention, he said. "But the system became too complex. Systems that last too long finish up by exhausting their virtues. In France, state intervention had become excessive."

France "has reached the end of a certain epoch, with both the right and the left acknowledging that we have reached the limits of the system" of state economic control. But he emphasized that his view of cohabitation did not include a vision of Socialist and conservative philosophies merging toward the center.

"We are not a government of national union. This is coexistence between institutions, between a president and a parliamentary majority" of differing parties that will continue to oppose each other's political programs, he said.

Asked whether these conflicts could lead to Mitterrand resigning and calling an early election, Balladur responded, "I don't see such a necessity. It is up to the president to make such a decision, but it cannot be a decision made for no reason."