Georges Bos, the chief of staff for the Nord-Pas-de-Calais governor, told The Associated Press that the spectator was rushed to a local hospital but that "his injuries are not life-threatening."
The accident happened in the small town of Saint-Floris, 69 kilometres into the stage between Orchies and Boulogne-Sur-Mer.
Bos added the vehicle driver was not injured but shocked.
The publicity caravan comprises 160 vehicles promoting 33 brands. All the vehicles are allowed to use the route of the race about one hour ahead of the peloton.
A boy was killed in 2000 and another in 2002 after being hit by sponsors' vehicles. Tour organizers responded by reducing the number of the vehicles allowed on the course.