As noted in the article linked by @Batman. French stems are .2 mm smaller than the 22.2mm 1 inch steerer quill standard.
Options are to replace the stem with a 22mm stem, sand down a 22.2mm stem to 22mm, or replace the fork (not recommended).
Article text follows:
Many higher-quality French bicycles of the '70's came with AVA brand
bars and stems. These have a reputation for failure, and should be
replaced. French stems differ both in the size that fits into the
steerer, and the size of the part that clamps on to the handlebar.
Thus, if you want to replace the handlebars on a French bicycle, you
will probably also need to replace the stem.
French stems are 0.2mm narrower than others where they fit into the
steering column. A standard 7/8" / 22.2mm stem won't usually fit. In
many cases, the limiting factor for fitting the stem in will be the
headset locknut, rather than the steerer itself. Try removing the
locknut before sanding down a stem. If the headset locknut is slightly
tighter than the steerer, it can be enlarged easily with a small
In cases where the stem really won't fit into the steerer, a few
minutes' work on the stem with sandpaper will usually do the trick.
Wrap the sandpaper around the stem, grip it with your hand, and turn
the stem round and round until it fits. You only need to remove 0.1
mm, which is 1/250", not much at all! [This needs to be a quill-type
stem, which inserts into the steerer tube, and an aluminum alloy stem,
not a steel one -- from which you would be removing chrome plating,
leaving it vulnerable to rust. Another approach is to replace the fork
and the headset with ones with standard British/ISO dimensions. This
is possible, as French head tube bearing race dimensions are 30.2mm,
the same as British. You could use a standard 22.2 mm quill stem, or a
threadless headset and a 1-inch Aheadset-type stem. Also, an adapter
is available as of 2014 to fit a 1 1/8" Aheadset-type stem to a French
fork.-- John Allen]
If you convert from a French stem to a standard one, you will also
need to replace the handlebars, which are a different diameter.