The man who advises Mitterand

France has deeply irritated the Reagan administration with its plans to sell been warning for several weeks now that the internal situation in Nicaragua is deteriorating, with an increase in the Soviet and Cuban presence, a local armed forces buildup that threatens to turn Nicaragua into a Central American ''superpower,'' and an erosion of individual liberties in the country.

Such warnings have not deterred the French. It is President Mitterrand's conviction that the United States, as much as the Soviet Union and Cuba, is the principal cause of class struggles and revolution in Central America. Since his election 10 months ago, Mitterrand has stated publicly on two occasions that France had a markedly different view of the third world than the US under Reagan and would pursue its own policies.

More than a few Frenchmen (and a few Americans) believe that the current Franco-American identity of views on East-West questions such as Poland and the need for nuclear arms parity in Europe is in reality little more than camouflage for profoundly differing views of policy toward revolutionary movements in the third world.

Given the personal history of Mitterrand's principal adviser on Latin America , Regis Debray, French policy is not surprising. Debray, 41, today occupies an upstairs office with gold-lame chairs in the presidential Elysee Palace, but 10 years ago he was languishing in a Bogota prison, having been captured shortly after the Bolivian Army caught, and shot, Ernesto (Che) Guevara. Guevara, a Cuban who had been Fidel Castro's right-hand man during the Cuban revolution, had tried to take the revolution into the Andes.

In several ways, Debray, a large-headed revolutionary with a Lech Walesa mustache, has tried to pattern his life after another French hero-intellectual, Andre Malraux. Like Malraux, Debray has attempted to wed thought and action, art and adventure. His writing, a mixture of fiction and nonfiction, attempts both to analyze and romanticize revolutionary movements. A novel, ''La Neige Brule,'' which won one of France's top literary prizes five years ago, starts in Cuba and ends in Bolivia. It romanticizes both Castro and Guevara. Debray's ''Conversations with Allende'' remains the best remaining testament on the Chilean socialist experiment under Salvador Allende. A book 10 years ago, ''Revolution Within the Revolution,'' and a new one, ''Critique de la Politique, '' are serious attempts at political analysis.

Just as Malraux found in de Gaulle a political figure able to translate his ideas into action, Debray has found one in Mitterrand.

The essence of the Socialist approach to the third world is simple: help those nationalist movements struggling against oligarchy and dictatorship so they are not driven into the arms of communism and the Soviet Union. Mitterrand, who during his years out of power visited both Cuba and Chile, is convinced that Castro turned toward Moscow because he was ostracized by Washington; that Allende failed because he became isolated. The French President believes that American attempts to shore up Somoza-type oligarchies in the region are doomed to failure, and that the West should instead take the lead in supporting reformist movements wherever they are found.

French policy toward El Salvador, while not involving arms exports, is similar. A joint Franco-Mexican statement last fall urged that the Salvadoran left be recognized as a ''legitimate political force'' and is defended by the French as an attempt to keep that movement from being driven into dependence on Havana and Moscow.

French officials admit the new policy carries risks, both for Nicaragua and in Franco-American relations. They say, however, that the French commitment to reform is deep enough to run the risk. If French policy fails - and Nicaragua falls farther under communist influence - the role of French policy in the failure will have been marginal. On the other hand, ''if we succeed, the gains will be enormous, for all of us.''

Latin America, said a French official, ''doesn't want to be dependent on Moscow. Moscow can give them only one form of aid - weapons. We have much more to offer. They know that.''