Rescuers used helicopters to evacuate people, including many children and tourists in three regions in the south of the country.
A 70-year-old German man who was helping to supervise children at a summer camp at Saint-Julien-de-Peyrolas in Gard is feared to have been swept away by floodwaters, together with with his van.
Torrential rain swelled rivers in the Gard region in southern France, flooding five campsites.
Trapped by a muddy, fast-flowing torrent after the Ardeche river burst its banks he sought shelter inside his caravan, but the vehicle was swept away.
Divers were taking part in the search for the missing man, an AFP journalist at the scene said.
The Gard government also said four German children were taken to hospital on Thursday suffering with hypothermia in Bagnols-sur-Ceze.
About 750 people, mostly from campsites, were evacuated in the Gard region alone, local authorities announced last night.
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The statement added that 119 children from a campsite in the town of Saint-Julien-de-Peyrolas in southern France had been "sheltered" from the floods.
'No one has suitcases.
About 400 firefighters and policemen, backed by four helicopters, helped in the evacuations and performed rescue operations, the French interior minister said.
UK Met Office meteorologist Marco Petagna said almost 3 inches of rain fell over the area in just 24 hours. Authorities warned that the flooding would take time to recede and urged people to be vigilant.
The French weather service has issued orange and yellow alerts for parts of south-east France that will remain in place until 4pm on Friday.
Heavy rain meant tents were destroyed, while about 17,000 homes were without power.