The conservative candidate has been given preliminary charges which allege he misused public funds and failed to properly declare his assets.
He will appear before a magistrate on Wednesday.
The 62-year-old is alleged to have paid hundreds of thousands of euros in taxpayers' money to his British-born wife Penelope for work she may not have carried out.
Two of the couple's children, Marie and Charles, have also been interviewed by investigators over claims they held "fake jobs".
Under French law, being put under formal investigation means there is "serious or consistent evidence" that points to probable involvement of a suspect in a crime.