It comes as Mr Cahuzac admitted to having had a foreign bank account.
The ex-minister had until now denied a report by investigative website Mediapart that he kept an undeclared Swiss bank account until 2010.
Mr Cahuzac - who gained a reputation as a vocal crusader against the use of overseas tax havens - quit in March.
Mr Cahuzac said he made the admission about the bank account during a meeting with investigating magistrates on Tuesday.
In a statement, he said he had about 600,000 euros (£509,000; $770,000) in the account.
He also asked for forgiveness from French President Francois Hollande and his former government colleagues.
"I was caught in a spiral of lies and lost my way. I am devastated by guilt," the statement said, quoted by the AFP news agency.
"It was an unspeakable mistake to think that I could avoid confronting a past that I wanted to consider behind me. I will now face this reality with all transparency," the statement said.
Mr Cahuzac had earlier insisted that he had done nothing wrong. He was a plastic surgeon before entering politics.
Correspondents say the scandal is embarrassing for the French Socialist government which has made fighting tax evasion a priority, as it tries to cut public debt.
Former French Budget Minister Jerome Cahuzac has been placed under formal investigation over alleged tax fraud, his lawyer has said.