I thought it would be easy but no.
I can't tell exactly and for sur what is the oldest restaurant in Paris.
La Tour d'Argent claims to have been founded in 1582.
But there are many doubt about the date if it existed it wasn't at its current location (this part wasn't even built at that time).
It only appears on document in 1860 at this place.
La Tour d'Argent, Quai de la Tournelle, Paris
La Bouteille d'Or opened in 1630
This restaurant is protected as a historical monument
It's currently closed. Looking for a buyer.
La Bouteille d'Or, Quai de Montebello, Paris
A La Petite Chaise opened in 1680 but there are some different claims (1700) and it wasn't a restaurant at that time but a wine cellar and grocery shop.
A La Petite Chaise, rue de Grenelle, Paris
Café Procope opened in 1686.
But the current Café Procope isn't the original exactly the one that closed in 1890 but it stand at the same place.
Café Procope, rue de l'Ancienne Comedie
What weird is about those is that there're all on the Left (south) bank of Paris when the most commercial part of the city is the Right (north) Bank. It is logically there that the first restaurants developed, especially when the main market of Paris was at Les Halles.
So I can't give for sure a specific restaurant and a specific date.
What I know is that the oldest (still existing) Japanese restaurant is
Takara of Paris opened in 1958.
Takara, rue Moliere
...And the oldest (still existing) Chinese restaurant of Paris the
Empire Celeste opened in 1953
1
Empire Celeste, Rue Royer-Collard953
20 years ago, I would have said the CHOU CHEN that served food between 1929 and 2003 until it's closure. It wasn't that far away.