Upvote:2
As long as the countries you are visiting are part of the Schengen treaty, you should be fine, you will only need a multiple entry visa if you will be going into a Schengen country then to another country which is not a member, an then in again.
In your case both Belgium and France are members of this treaty, so again, you won't need a multi-entry visa.
If you need to be more confident about this, you could consult with the French Embassy in your country of residence.