The bus was carrying 75 Czech tourists from Spain to the Czech Republic when the incident described as an "isolated act" occurred on a highway on Saturday night.
"An initial shot smashed the front windscreen and a second exploded the rear window of the vehicle a few seconds later," said prosecutor Valence Alex Perrin.
Six passengers were hurt by broken glass, including a woman whose eye was seriously damaged, and a child.
Perrin said initial investigations had revealed the shooter used a hunting rifle. One of the bullets was found inside the bus.
All the passengers who were not hospitalised have returned to their home country.