Henry Kissinger's "unofficial" fact-finding trip to the Middle East has confronted Ronald Reagan with this delicate problem: how does the incoming Republican administration prevent the itinerary and pronouncements of one of the world's leading statesmen from having an unwanted effect on U.S. foreign policy?

The problem is not inconsequential. Reagan is committed to returning American diplomacy to its traditional forms. He wants his secretary of state to be the nation's chief diplomat in substance as well as form. He wants U.S. ambassadors to play the leading role as the president's eyes and ears abroad, and he intends to downgrade the influence of the National Security Council staff over foreign policy.

"Reagan has never wanted any grandstanders in his diplomacy," one Reagan insider told us. "But here is Henry holding daily press conferences about how Reagan intends to settle the Middle East, and using his own agents to get invitations from countries like Oman that think he really is doing Reagan's bidding."

In Kissinger's defense, his prestige and stature around the world are not much below the peak years of the Nixon-Ford administration. To the world, he is the most glittering figure in America, sought out by statesmen for confidential advice and talks. That role, which he plays with consummate skill, was particularly suited for an opposition voice in a Democratic administration. Under Republican Reagan, however, it is particularly unsuited -- unless Reagan should bring him into the center of his foreign policy machinery. That is not in prospect today.

Kissinger himself was well aware of the change in his status wrought by Reagan's election. What has now become his "unofficial" fact-finding tour was planned last summer as a pleasure junket to Egypt and the Upper Nile with old friend Bill Paley. Ronald Reagan's election gave it new contours. It suddenly became a target of opportunity for Kissinger both to do what he does so well -- negotiate between contending parties -- and to demonstrate his talent and usefulness directly to the new president.

But Kissinger himself was aware of the amenities. He called Secretary of State-designate. Alexander Haig, NSC Director-designate Richard V. Allen and Sen. Paul Laxalt, Reagan's closest conressional confidant, to tell of his plans. He told Haig that if his trip would in any way discomfit Haig or the president-elect, he could cancel it. But cancellation would have made Reagan and Haig appear unduly insecure and was not even considered.

Kissinger was told that he should proceed with his vacation tour but must understand that he carried no portfolio from Reagan. In effect, both Haig and Allen told Kissinger what Reagan had said publicly that he told kissinger in an offhand way long before his trip: have fun, Henry, and if you run into anything interesting, do give us a ring.

Kissinger's itinerary was then enlarged. He wanted to add Oman, the strategic state that sits at the entrance to the Persian Gulf, but had not been invited. The invitation was procured for him by Philip Habib, an old Kissinger friend from State Department days, who got it while in Oman as part of the U.S. delegation to the 10th Omani anniversary celebration in November. A close Kissinger ally in Washington with good Omani credentials also helped.

But the Omanis were uneasy. They were led to believe that Kissinger's visit had Reagan's specific blessing, but there was no letter from the president-elect. In Jordan, King Hussein carefully ascertained that Kissinger's trip was not reaganized and declined to receive him. Egyptian and Israeli officials, also concerned about Kissinger's credentials, were politely informed by Reagan agents Kissinger was strictly on his own with no Reagan imprimatur, despite what on Reagan aide called "the aura of a close connection with Reagan" that would be dropped along the way.

The full measure of Henry Kissinger's vestigial influence, however, is clearly visible in the tightly controlled way Reagan's foreign policy team has handled his "unofficial" Mideast tour with its daily press conferences and hints of Reagan-Kissinger proximity. There has been no overt dissociation with the brilliant former secretary of state, no snide public asides about his motives.

Nor is there any effort to conceal the reason for this among Reagan insiders. "Henry is a valuable and important potential asset to us," one top-level Reagan adviser told us. "He towers above the world pack and at the right time President Reagan may need his services, maybe in Europe, maybe in the Middle East, maybe in the Kremlin. We don't want any hint of alienation between Reagan and Henry."

But another "unofficial" tour like the current private-shuttle diplomacy through the Middle East could estrange Reagan and Kissinger, undermining his potential utility as the unique national asset Reagan -- and the world at large -- perceive him to be today.