MONTE CARLO, Monaco (AP) _ Jules-Francois Crahay, a Belgian-born couturier credited with keeping the long-established Lanvin house among the leaders of Paris fashion for two decades, has died, officials said.

Crahay was found dead of cardiac arrest in his apartment here on Tuesday, the principality ambulance service reported. He was 70. Burial was scheduled Saturday in the southern French village of Bagnols en Foret.

Crahay joined the prestigious Lanvin firm in 1963 and stayed until his retirement 20 years later. He succeeded Antonio del Castillo, who had designed its collection from 1950. Founder Jeanne Lanvin died in 1946.

Crahay's first major outfit for Lanvin to hit the headlines was the wedding dress for Maryll, who married Bernard Lanvin, later president of the company's perfume side. Maryll launched her own Lanvin ready-to-wear collection in 1982.

Not known as a revolutionary in fashion, Crahay was admired for his sumptuous, ladylike outfits that dazzled with opulent fabrics. While he usually made fairly long dresses, he also showed sexy bloomers and tight strapless tops for evening wear.

''I love folklore,'' he once said in an interview, and many of his clothes had more than a touch of the rich peasant, complete with shimmering stoles and shawls.

He moved outside the traditional circles in choosing his models. One of his choices for the 1982 season was cellist Olga Rostropovitch, who sashayed down the runway in dressy peasant attire under the beaming gaze of her father, master cellist Mstislav, and his wife Galina Vishnevskaya.

Crahay was born May 21, 1917 in Liege, Belgium. His father was an industrialist and his mother a couturiere, who encouraged her son's talent for drawing and design. In 1934 and 1935, he studied design and cutting at a Paris couture school, returning to Liege to work for his mother.

Drafted into military service in 1938, Crahay was taken prisoner by the Germans in 1940 and was not released until 1945, when he returned to Liege and took over the family couture business.

Though the business was successful, Crahay yearned for Paris and went there in 1951 to start a partnership with Germaine de Vilmorin.

Following financial problems, Crahay accepted a job with Nina Ricci and made a hit with his 1959 collection.

He was immediately recognized with a prize by Filene's of Boston in 1959, and by Neiman Marcus in 1962. When the House of Lanvin lured him into becoming its artistic director, he made a big hit with his first major collection there in 1964.

After his retirement, he lived mostly in Monte Carlo.