The French press, viewing western economic summits as a kind of annual political football game, has concluded that this year's match was won by President Francois Mitterrand.

It was lost, in the view of French commentators, by Prime Minister Jacques Chirac. Ronald Reagan, Margaret Thatcher, Yasuhiro Nakasone and the other summiteers were simply spectators.

A sideshow for the rest of the world, the political battle between the Socialist head of state and the conservative prime minister took center stage here. It was seen as a major test of "cohabitation" -- an unprecedented power-sharing arrangement that resulted from a narrow right-wing victory in parliamentary elections on March 16.

Mitterrand's success at the Tokyo summit, where he was treated as France's chief spokesman, was some recompense for a series of political setbacks over the past two months. As far as French domestic policy is concerned, it is Chirac who has mostly carried the ball as head of the new right-wing government.

The president, whose mandate does not expire until 1988, has been forced to watch passively as day-to-day executive power drifts inexorably toward the prime minister's office on the opposite bank of the Seine River. His advisers, who were once key players in the machinery of government, are now reported to have little to do.

An energetic politician, Chirac has made clear that his constitutional powers in office extend to foreign policy, an area regarded as private property by French presidents since Gen. Charles de Gaulle. Chirac has formed a staff of high-powered diplomatic advisers and visited several foreign countries, including Britain, West Germany and the Ivory Coast since taking office.

In theory, Chirac's trip to Tokyo was designed to demonstrate that the government is responsible for foreign policy. In practice, protocol constraints and summit traditions meant that the prime minister was effectively upstaged by the president.

"Master of the game in Paris, Chirac became a marginal figure in Tokyo. Diminished in France, Mitterrand temporarily recovered the attributes and powers of his position -- for the time that he was abroad," noted the Paris weekly, L'Express.

After the Japanese hosts made clear that there was room for only one French leader at the state dinner on Sunday night, Chirac diplomatically announced that he would arrive in Tokyo on Monday afternoon aboard a regular Air France flight. It seemed a neat solution to the protocol problem, but it meant he was absent for much of the important political business of the summit.

The joint declarations on terrorism and nuclear safety were drafted before Chirac arrived in the Japanese capital, leaving him with nothing to do except approve the contents. When Mitterrand addressed a press conference on the final day of the summit, Chirac sat off to one side, smiling but not opening his mouth.

Mitterrand's victory was more symbolic than substantive; there is little real difference between the two leaders over the handling of these foreign policy issues, anyway. During the past few weeks, both men have sought to coordinate their positions on such issues as terrorism in order to prevent France from being ridiculed by its western allies.

In an apparent attempt to recoup, Chirac held a private meeting with Nakasone during which he discussed the subtleties of "cohabitation." He told his host that France was following economic policies that were the "exact opposite of those pursued for the past five years."

Chirac's point, stressing the political differences between himself and Mitterrand, appeared to be lost on Nakasone, who compared "cohabitation" to the relationship between "a young husband and wife who still do not know each other very well." An account of the conversation was leaked to reporters by Japanese officials.

Ingrained French suspicions of the United States surfaced before the summit when several officials suggested privately that the Reagan administration might try to take advantage of "cohabitation" by playing one leader off against the other. France, it was said, had to speak with "one voice" even if it came out of "two mouths."

As is often the case, the French seem to have overestimated Washington's capacity for intrigue. The official White House spokesman, Larry Speakes, demonstrated the extent of his grasp of the French constitution by referring in a news conference to "Prime Minister Mitterrand and Foreign Minister Chirac."

The reaction in France was a mixture of amusement and relief.