Metal Detecting in France
Information about the law apertaining to France can be obtained from the NCMD site listed on the Useful sites page or take this link direct to the relevant pages.
In broad terms the law says that to search on any land be it your own or someone elses, not only do you have to have the landowners permission in writing, but by virtue of the fact that you are searching for objects which could concern/interest prehistory, art or archaeology you are required to obtain administrative authority from the Prefect of the Region. This authority comes in the form of a license which may be issued after reference to the searchers identity, competance, experience and method of searching. Regard is also taken of the location, scientific objective and the duration of the searches to be undertaken.
Failure to take notice and act upon this could result in the confiscation of the metal detector being used and a fine.
It is believed that beach detecting falls outside of this law.
If your French is good enough you may be able to glean more info from the following site:
http://www.tresors.com.fr/ When you get on the home page scroll down to the link to la-Loi.