Didier Lombard, who ran the company between 2005 and 2010, was told on Wednesday he was being investigated for workplace harassment and put on bail of €100,000 ($125,000), his lawyer Jean Veil said.
Lombard was the first person to be heard by Pascal Gand, the investigating magistrate handling the case, said Veil.
But other senior executives, including the company's former head of human resources Olivier Barberot, and Louis-Pierre Wenes, formerly number two at France Telecom, are due before the magistrate Thursday.
Representatives of the company itself will face the magistrate on Friday and may be told that the company has also been placed under investigation.
Lombard stepped down from his post in March 2010 after a series of 35 suicides among employees of the company between 2008 and 2009.
During his time in charge, Lombard oversaw a reorganisation that involved the loss of 22,000 jobs between 2006 and 2008. In addition, more than 10,000 employees were switched to other jobs.
A statement from the SUD union, which in 2009 filed a complaint over the suicides, welcomed news about the Lombard probe.
"It is the first time in France that a former business leader ... has been placed under investigation for moral and institutional harassment," it said in a statement.
Sebastien Crozier of the CFE-CGC union also welcomed the development.
"It's important for all the staff and the families," he said.
The union's lawyer, Frederic Benoist, said the way the redundancies programme had been rushed through had put undue pressure on staff.
Lombard, writing in the French newspaper Le Monde, has denied that the way the reorganisation was handled had contributed to the suicides.
But a 2010 report by work inspectors highlighted what it said was management harassment of the white-collar staff in particular, many of whom had been sidelined, urged to accept a career change or to leave the company.
France Telecom's management methods had effectively undermined staff psychologically "undermining their physical and mental health", the report said.
Work inspector Sylvie Cattala wrote in a letter to a branch of the SUD union that between 2005 and 2009 management had repeatedly been warned of the dangers.
The formal investigation into the deaths at France Telecom was opened in April 2010 – the month after Lombard stepped down.
In April, police entered France Telecom's Paris headquarters to seize documents as part of that investigation.
France Telecom is one of the biggest companies in France, employing 105,000 people in France and 171,000 worldwide.