Strauss-Kahn, the former head of the International Monetary Fund, is accused of trying to sexually assault a writer during a 2003 interview.
Writer Tristane Banon accuses him of attacking her during a 2003 interview for a book she was writing. She claims Strauss-Kahn grabbed her hand and arm before the two fell to the floor of his apartment and fought for several minutes, with the politician trying to open her jeans and bra and putting his fingers in her mouth and underwear.
His lawyers call the incident "imaginary."
The investigation will help prosecutors determine whether her allegations support a charge of attempted rape or sexual assault. Under French law, sexual assault has a three-year statute of limitations - meaning the 2003 incident would be too old to prosecute - compared with a 10 year limit for attempted rape.
Strauss-Kahn - a Socialist politician who had widely been considered a leading contender in next year's presidential race - returned to his native France about a week ago after months in New York battling the rape charges brought forward by a Guinean immigrant.
Those charges were dropped. Strauss-Kahn resigned from his job as director of the IMF during the media storm that erupted when the case became public on May 14.
Strauss-Kahn still faces a lawsuit filed by the New York hotel maid, Guinean immigrant Nafissatou Diallo, who says he forced her to perform oral sex and tried to rape her when she came to clean his hotel room. Through lawyers, he has acknowledged that a sexual encounter took place but contended that it was consensual.
The Associated Press contributed to this report.