WHEN PRESIDENT REAGAN goes to Canada next week his hosts will ask him, with a shade of apprehension, about that gas pipeline from Alaska. The route traverses Canada, and the Canadians are hard at work building their sections. But the United States has yet to turn the first spadeful of earth. While the Canadians are not precisely uneasy, they would like to hear the new president repeat the promises that his predecessor absolutely and categorically provided.

But perhaps Mr. Reagan as well will feel a twinge of anxiety as he contemplates the question. The pipeline would be roughly three times as long as the Alaskan oil line that was, until this point, the largest construction project every undertaken. It would start at the same place, the gigantic oil field at Prudhoe Bay, but would descend across the Yukon Territory and Alberta toward, as Alaskans say, the Lower 48. The cost? It's hard to say, but perhaps $25 billion. Under the last administration, the obvious questions of financing were never resolved. They now fall to Mr. Reagan, who will perceive that they are on a scale sufficient, if mishandled, to have an impact on his own economic plans.

The Canadians will probably also want an answer to their requests for an agreement between the two governments on pricing. The conservation will remind Mr. Reagan that he does not yet have an energy policy that goes beyond generalizations. Nor has he appointed anyone at the Energy Department capable of dealing with this kind of highly specific negotiation.

The Prudhoe Bay gas will be expensive when it arrives at the midwestern and northwestern terminals. The cost of building the pipeline makes that inevitable. Under present circumstances, it would not necessarily be difficult to sell that gas, if the distribution companies were permitted to average its cost in with that of the cheaper gas being sold under price regulations.The regulations now hold the average price of gas at the well to about-one-fourth the cost of its equivalent in oil. If the Reagan administration were successful in persuading Congress to deregulate gas, that margine of financial safety for the pipeline would disappear.

But the Alaskan gas pipeline ought to be built. Although the risks and the doubts are real and worth noting, the decision has already been made. The United States has committed itself to the project. It cannot change its mind now without severely damaging its standing in Canada, and diminishing the value of pledges on more subjects than energy. The cost will probably rise, but the basic logic is unimpaired. If the gas is not necessary immediately, it will be badly needed within a few years. Like all the other dollars spent on alternative sources of fuel, the investment in this pipeline will be insurance against further leaps in oil costs and a countermeasure against the economic power of the oil exporters. The president's trip to Canada will serve a useful purpose if it focuses his attention on future fuel supplies -- not the least important of which is the Prudhoe Bay gas.