Bernard Phelan, a Franco-Irish citizen detained in Iran, must be released immediately and provided access to urgent medical care, the Foreign Ministry in Paris said on Thursday.
"The denial of medical access by Iranian authorities is unacceptable", Foreign Ministry spokesperson Anne-Claire Legendre told journalists, adding France was very concerned about Phelan's poor health condition.
Phelan, a Paris-based tourism consultant, was jailed in Iran in October.
Phelan is being held in Vakilabad Prison in Mashhad city on multiple charges, including disseminating anti-regime propaganda and taking pictures of security services.
Phelan has denied all the charges.
The family of Louis Arnaud, a 35-year-old French national, who had been in Iran on tourism, on Thursday made public that their son had been arrested on Sept. 28 and was being held in Tehran's notorious Evin prison.
In recent years, Iran's elite Revolutionary Guards have arrested dozens of dual nationals and foreigners, mostly on charges related to espionage and security.
Rights groups have accused Iran of trying to extract concessions from other countries through such arrests. Iran, which does not recognize dual nationality, denies taking prisoners to gain diplomatic leverage.
French Foreign Minister Catherine Colonna on Wednesday spoke to her Iranian counterpart Hossein Amirabdollahian demanding the release of all the prisoners.
Some 40 Europeans are held in Iran. Speaking in Brussels on Monday, Colonna said she wanted a collective European response to deal with the matter.