The fraud case involves the FN and Riwal, a company run by some of its members.
They are suspected of inflating costs so as to claim unjustifiably large sums in the state subsidies given to parties' election campaign expenses.
Le Pen was questioned all morning on Tuesday, according to sources.
She refused to turn up to two earlier interviews last Autumn.
The investigation started in 2014 and the FN claims it is politically motivated.
Separate investigations are looking into charges that Marine Le Pen and her father Jean-Marie, who founded the party, underestimated the extent of their personal wealth in compulsory declarations for elected representatives in 2014.