But under the clever supervision of Billy Wilder, who helped to write the script, then produced and directed "The Apartment," which opened at the Astor and the Plaza yesterday, the idea is run into a gleeful, tender and even sentimental film. And it is kept on the side of taste and humor by the grand performance of Jack Lemmon in the principal role.

This Mr. Lemmon, whose stock went zooming last year with "Some Like It Hot," takes precedence as our top comedian by virtue of his work in this film. As the innocent and amiable young bachelor who methodically passes around the key of his modest brownstone-front apartment among the sultans of the place where he is employed, he beautifully maintains the appearance of a lamb among ravening wolves. He has the air of a good-natured hermit who calls Grand Central Station his home.

His character does not like what he's doing. He would much prefer to stay in his bed on a rainy night when a sozzled sales executive telephones and demands the key. But he turns out, in line of duty, when the hint of a promotion is flung, and he continued to oblige, confidentially, until the inevitably romantic trouble brews.

You can probably guess the reason. It is one of the elevator girls, for whom our fellow has worked up quite a fancy but whom he discovers is using the apartment with the head of personnel. Then he goes through an ordeal of worrying, especially after the girl has the rashness to choose the apartment for a suicide attempt on Christmas Eve. That makes for a sticky situation and a sharply ironic point of view on the perfidiousness of men with families playing around with the office girls.

Even in this dismal incident, Mr. Wilder and his co-author on the script, I. A. L. Diamond, have managed to keep the action and the dialogue tumbling with wit. In the midst of a grim operation to get a pill-poisoned girl to come awake, they relieve the graveyard tension with trenchant and credible gags. And they bring the sentiment to focus with a wistful remark from the girl. "When you're in love with a married man, you shouldn't wear mascara," she says.

Mr. Wilder has done more than write the film. His direction is ingenious and sure, sparkled by brilliant little touches and kept to a tight, sardonic line. In addition to Mr. Lemmon's, there's a splendid performance by Shirley MacLaine, as the daffy girl who gets into a lot of trouble, and a good one by Fred MacMurray, as the wicked boss. Jack Kruschen makes a funny doctor-neighbor who mistakes Mr. Lemmon for a ladies' man, and Ray Walston and David Lewis are amusing (and slightly sordid) wolves.

Adolph Deutsch has contributed a light, sentimental accompanying score, and Joseph LaShelle, the cinema-photoger, has made the whole thing look quite stylish and metropolitan on the black-and-white large screen.