I'm back from Côte d'Azure.
Thanks to the tecnique mentioned above I was having a quite nice mobile connention.
I bought a SIM card with €15 in advance from the seller and created an account on Orange. Differrent from the instraction, the SMS for password was free, so that I could contract Internet option(€9) and Mial Option(€6) at once.
I noticed that, if you don't email heavily, it would be a nice tactics to hold Mail Option until entering France. In detail, buy SIM card with €10 and contract Internet Option alone, nagate mail sync, and buy €5 charge in France and contract Mail Option. This will save a bit.
Anyway, thx Sirocco!