Yves Cohen said negative tests for flu and other coronaviruses on 24 patients in hospital with respiratory problems during December and January had been checked.
And in a development experts say would be of huge significance, one of the revisited samples came back positive for COVID-19.
Dr Cohen, head of resuscitation at a Paris hospital, said the test had been repeated a number of times in case of error.
He told France's BMTV that medical staff had then phoned the man to discuss the new finding.
"He was sick for 15 days and he infected his two children, but not his wife, who works in a supermarket, at the fish stall," he said.
"We wondered [she worked with] fish of Chinese origin, but she only works on French products.
"Then we learned by chance that she worked alongside sushi sales or people of Chinese origin."
He suggested that the man's wife may have had COVID-19 but not shown any symptoms, adding that it was the responsibility of other agencies to investigate further.
