The probe centres on claims the centre-right UMP party - which Sarkozy now heads up - should not have paid fines levelled against the former French leader after he broke campaign spending limits.
The UMP is accused of using public cash to pay off Sarkozy's fines, and the party's former chief Jean-Francois Cope has been charged over the affair, as has former party treasurer Catherine Vautrin.
The former president arrived at the financial section of the main Paris court today morning for questioning by investigating judges.
Sarkozy denies any knowledge of, or role in, the apparent fraud which meant his campaign spent nearly 50 per cent more than it was legally entitled to.
This is just one of several legal woes that risk derailing Sarkozy's comeback, which saw him elected to the head of the UMP last autumn ahead of another possible run for the presidency in 2017.
But they have so far had little impact, with the UMP winning a thumping victory in weekend local elections.