The monumental fresco cycle entitled "The Loves of the Gods" was painted at the end of the 16th century by the artist Annibale Carracci in the Palazzo Farnese.
"For art historians, this is still one of the most perfect examples of Renaissance painting and sculpture," Erkki Maillard, the embassy's second-in-command, told AFP.
"This is kind of our Sistine Chapel!" he said.
The work is being financed by the World Monuments Fund for Europe, a private non-profit organisation.
The fund's president, Bertrand du Vignaud, said there were "a huge number of cracks" in the frescos and the cycle needed to be cleaned up and strengthened.
The gallery has been restored several times but this is the first time that the side walls — not just the vaulted ceiling — will be restored.
The project was first launched four years ago, with experts debating how the work should be carried out.
It will cost €1.0 million ($1.4 million), with €800,000 coming from WMF through private donors and €200,000 from the Italian government.