Messina Denaro, caught on Monday at a private hospital in Palermo after 30 years on the run, is being held in the central Italian city of L'Aquila, the Palermo prosecutor said, after being transferred from Sicily on the day of his arrest.
The apartment is located in an a modest building near the centre of Campobello di Mazara, a town in the Western Sicilian province of Trapani, just a few kilometres from Messina Denaro's home town of Castelvetrano.
The house is believed to have been used continuously in recent weeks but investigators are still searching for other places where the mafia boss might have spent time.
"In this case, it is a house which allows us to have an idea of how the boss lived, and he lived well," Palermo prosecutor Maurizio De Lucia said.
Messina Denaro was known for his taste for luxury goods, including sun glasses and clothes. Police said he was wearing a watch worth 35,000 euros ($38,000) when he was arrested.
Nicknamed "'U Siccu" (The Skinny One), Messina Denaro picked up 20 life prison terms in trials held in absentia for his role in an array of mob murders, including the bomb attacks that killed anti-mafia prosecutors Giovanni Falcone and Paolo Borsellino in 1992.
Investigators believe Messina Denaro was driven on Monday to Palermo's La Maddalena hospital from Campobello di Mazara to be treated for cancer. The town was home to his alleged aide Giovanni Luppino, who was arrested with him.
Despite his illness, prosecutors said Messina Denaro was fit enough to serve time in prison where he will carry on with his cancer treatment. ($1 = 0.9232 euros)
(Additional reporting and writing by Angelo Amante in RomeEditing by Keith Weir)
By Wladimir Pantaleone
