The decision to charge the defendant with sexual assault, rather than the more serious crime of rape, caused an outcry in September when it first emerged and led to public debate about the age of consent.
The government has vowed to introduce one for the first time because France does not treat sex between an adult and a minor as rape unless there is proof that force was used.
“There is no question of consent when we're talking about a child of 11 years,” lawyer Carine Diebolt, who is representing the girl and her parents, told reporters before the trial opened on Tuesday.
She has asked for the case in the town of Pontoise to be adjourned and a rape charge to be applied, insisting that the child was shocked, intimidated and threatened by the defendant.
The father of two children, now aged 29, approached the girl in a public area of a housing estate in Montmagny northwest of Paris. She performed oral sex in an elevator on the way to his apartment where they had penetrative sex.
His lawyer, Marc Goudarzian, cast doubt on the girl's testimony and insisted his client thought the girl was 17 because she had passed puberty.
“She wasn't born yesterday,” Goudarzian added.
In November last year, a man was acquitted of rape after having sex with an 11-year-old after a jury found no evidence that she had been forced into the relationship.
And in another widely publicised case, a teacher was given a suspended jail sentence, not a prison term, in the same month for having sex with a 14-year-old pupil.
President Emmanuel Macron, who is married to his former school teacher, has proposed a new law that would make 15 the age of consent