Prime Minister Menahem Begin of Israel made clear today that his Middle East peace plan is not a rigid blueprint but the opening gambit in a game of diplomatic chess.

"It is a proposal," he stressed to reporters at an airport press conference. "A proposal can be negotiated."

If President Anwar Sadat of Egypt has counterproposals, we will consider them," Begin said.

The two men are to meet Sudnay in Ismailia, in a renewed effort to settle disputes that have torn apart the Middle East for 30 years.

Begin declined to discuss details of his proposal. Nevertheless, in broad terms it is understood to offer Israel's withdrawal from the Sinai and provide some form of self-rule for Arabs living on the Israeli-occupied West Bank of the Jordan River.

Begin spent several hours here earlier today describing his plan to Prime Minister James Callagham and an envoy from President Valery Giscard d'Estaing of France.

Relaxed, smiling and making quips to reporters, Begin was unruffled by a suggestion that President Sadat would seek major amendments to the Israeli proposals.

"We shall meet like friends," Begin replied. "He says I am his friends so I can say he is my friend."

Begin added that American as well as British leaders have now approved his proposal as "a fair basis for negotiations conductive to peace." He recited a list of endorsements that included not only President Carter and Callaghan but also U.S. congressional leaders, former President Gerald Ford and Henry Kissinger, the ex-secretary of state.

Begin also disclosed that new moves are afoot to repair Israel's frayed relations with France, at a low ebb since President Charles de Gaulle denounced Tel Aviv's decision to strike first in the 1967 Arab-Israeli war.

Giscard, the prime minister said, had invited him to lunch on Wednesday. Begin, however, must report to his Cabinet on his talks with President Carter in Washington.

Begin said he had told Giscard's envoy here, Jean-Francois Poncet, that he hoped "we shall now go into a period of improved relations."

Begin has visited here twice in the past two weeks and diplomatic observers believe that this is a quiet reminder to Giscard tha an Israeli prime minister has his domestic political uses. The visists enhance Callanghan's figure as a global statesman and strengthen his position within the small Jewish community.

The Jewish vote in France is far more significant and that country holds a national election next year. The luncheon invitation from Giscard is seen as a reflection of these facts.

Begin, perhaps playing hard to get, said "no thanks now," but suggested that he hoped for a rain check.