Francis Evrard was charged with raping the boy within weeks of his release from prison, where he had served 18 years for a rape conviction.
Evrard, who has eight convictions for sexual crimes, admitted the first day of his trial that he raped the boy, The Times of London reported Tuesday.
"Yes, I did it," Evrard said Monday in court. "I cannot say why I have done that to other children."
In a letter last month to President Nicholas Sarkozy, Evrard asked to be castrated.
"At my age, I would not suffer and that will prevent my tendencies towards children," Evrard wrote.
The case has prompted French lawmakers to consider physical castration, illegal under current French law. Evrard's case also sparked public outrage when it was learned he had been prescribed Viagra during his 18-year prison stay, The Times reported.