In a case that could wreck the 57-year-old's hopes of a political comeback, Sarkozy is suspected of taking financial advantage of elderly L'Oreal heiress Liliane Bettencourt at a time when she was too frail to fully understand what she was doing.
Examining magistrate Jean-Michel Gentil is expected to spend most of the day quizzing Sarkozy about how he obtained funding from Bettencourt.
Judicial sources have said the 57-year-old could be formally indicted on a charge of taking advantage of someone in a position of weakness, although the magistrate also has the option of interrogating him as a witness under caution.
Bettencourt is now 90 years old and has been in poor health since 2006.
The allegation against Sarkozy is two-fold: That the money obtained from her took his campaign financing over legal limits and that it had been secured without her full knowledge or consent.
This latter claim was made by Bettencourt's former accountant, Claire Thibout in 2010. She told police that she had handed €150 000 in cash to Bettencourt's right-hand man, Patrice de Maistre, on the understanding it was to be passed on to Sarkozy's campaign treasurer, Eric Woerth.
Maistre, one of the biggest backers of Sarkozy's UMP party, withdrew a total of €4m in cash from Bettencourt's Swiss bank account in seven instalments between 2007 and 2009.
Investigators suspect some or all of the money could have found its way to Sarkozy or his party.
In July, the examining magistrate ordered the seizure of Sarkozy's diaries in order to establish what he was doing around the time the cash was being moved.
French authorities confirmed this week they have also opened a preliminary investigation over the management of opinion polls carried out during Sarkozy's 2007-12 term as president.
That probe was triggered by anti-graft organisation Anticor, which suspects the former president of having handed the contract for the polls to a company, Publifact, run by his former advisor Patrick Buisson, and of using public funds to carry out his own party political electoral research.
Sarkozy, who was in London on Wednesday giving a lecture to business figures, has not ruled out another tilt at the presidency in 2017. Aides refused to comment on either case.
Separately, Sarkozy is pursuing a defamation suit against online news website Mediapart over a report that former Libyan dictator Muammar Gaddafi provided €50m for his 2007 campaign.