Our study's purpose is to show the efficacy of the transcranial magnetic stimulation (a non invasive method of cerebral modulation) in patients suffering from chronic obsessive compulsive disorder (OCD). This new method will be applied in 20 patients during 4 weeks (5 sessions each week), and its effects on OCD symptoms will be compared to those of a "sham" (=placebo) stimulation applied with the same process in 20 other patients, randomly assigned to the comparison group. The maintenance of the therapeutic effects will be explored during 8 weeks following the end of the treatment. In addition to classical scales used to measure the treatment effects, all patients will be examined using a functional magnetic resonance imaging (fMRI) before and after treatment to explore the cerebral effects of rTMS