Are you using the original Nintendo-branded SCART cable, as supplied with your French SNES?
If so, you need to modify it slightly. Since the Nintendo cable is hard to disassemble, it may be a better idea to buy a third-party lead and use that.
The Nintendo cable properly implements function switching. However, this will not work properly with a Super Famicom. That should not be a problem though; just manually switch your TV to the SCART input when using the console.
You need to put capacitors in between the R, G, B and composite video signal wires and the SCART plug pins. 220uF electrolytic capacitors will work. Connect the negative terminal to the SCART connector pin, the positive terminal to the wire.
Rather than modify the Nintendo SCART cable (which would mean it no longer works with a PAL SNES), you can buy a SCART plug-to-socket cable, and install the capacitors in that. Then connect that cable in between the SNES SCART cable and the TV.
Perhaps the simplest option would be to fit a 50/60Hz switch to your PAL SNES and not use the Super Famicom at all. (Unless you have one of the few games that only works on an NTSC console.)