This summer, Mr. Veyrat — famous for his plant-based cuisine and for wearing a black shepherd’s hat — asked to be removed from the guide altogether. And on Monday, he said that he was suing the guide to “shed light on the exact reasons” for the downgrading, according to his lawyer.

“If I made a mistake in my cuisine, I immediately take responsibility for it,” Mr. Veyrat told Franceinfo. “If you come tell me that this thing wasn’t right, I take responsibility, and then take away my star. But there is nothing to take responsibility for, because we know nothing, we don’t know what happened.”

In an interview this summer with Le Point, Mr. Veyrat acknowledged that the downgrading had not affected his business — revenue was up 10 percent from last year, he said — and that his anger stemmed mostly from his wounded pride.

“I feel like my parents have died a second time,” he told the magazine. “Can you imagine the shame I feel: I am the only chef in history to get a third star and then to lose it the next year.”

Mr. Veyrat, through his lawyer, declined an interview request for this article.

The decisions of the red-covered Michelin Guide are closely watched in France. Every year, the 20 to 30 restaurants that get its top accolade, a three-star rating, are scrutinized to see which chefs are on the rise and which are losing their shine. This year, 27 restaurants were awarded three stars, and 84 were awarded two.