Frédéric Châtillon was arrested on 23 January and has been charged with fraud and money-laundering over the financing of Front National election campaigns in 2011 and 2012.
Châtillon claims the top secret investigation was politically motivated.
The investigation has looked into the running of a front organisation, Jeanne, set up to raise funds for FN election campaigns and Châtillon's business, Riwal.
Riwal received payment for services rendered during Le Pen’s 2012 presidential bid.
According to online investigative website Mediapart, judges have also been investigating Châtillon's alleged financial links to the Syrian regime of Bashar al-Assad, which Le Pen has refrained from criticising during the bloody civil war that has hit the country, although that inquiry was closed in April 2012.