Bernard Squarcini says he was acting in the national interest by tapping into the phone records of Gérard Davet, a journalist with French newspaper Le Monde.
Investigators though say his defence cannot possibly hold and consider him guilty of illegally collecting personal data.
Squarcini was trying to find the journalist's source in an investigative report on Liliane Bettencourt, France's wealthiest woman.
In 2010 Le Monde published a story that led to allegations of illegal funding from Bettencourt for the
election campaign that won Sarkozy the presidency.
If found guilty, Squarcini faces up to five years in prison and 300,000 euros in fines.
Considered close to Sarkozy, who appointed him to France's top intelligence post, Squarcini is a career cop famous for cracking down on the Corsican mafia.
He was sacked when Sarkozy lost the presidency to François Hollande and since then has been running a private consulting firm.