Nine Impressionist paintings that were stolen from the Marmottan Museum in Paris in 1987 by five gunmen are hanging at the museum once more. The paintings, including Monet's "Impression, Soleil Levant" and other masterpieces by Monet, Renoir, Morisot and Sei-Ichi Naruse (a Japanese who lived in France from 1921 to 1923), were recovered on the French island of Corsica last December and went on display this month. They were found in a quiet villa in Corsica where the paintings had been hidden.

No arrests have been made, but the police say they believe they know the identity of the thieves. Far from being professional art thieves, the police said, they are rather ordinary robbers who were unable to strike a deal with Japanese art collectors.

"Impression, Soleil Levant" (Impression, Sunrise), painted by Monet in Le Havre in 1872, is a landmark of art history. An art critic, seeing the painting on display in 1875, ironically labeled Monet and fellow exhibitors "Impressionists," a word that stuck to the budding art movement.

The nine paintings are all in good condition and have been cleaned, some for the first time. The varnish that coated "Impression, Soleil Levant," added after the painter's death, has been removed, thus restoring the painting to its original tones of grays, pinks and yellows.

The Marmottan Museum, at 2 Rue Louis-Boilly in the 16th arrondissement, is open daily except Monday from 10 to 5:30 P.M.; telephone Admission is about $4.50.

Photo: The Marmottan, where the nine stolen paintings hang again. (Academie des Beaux-Arts/Institut de France)