The NEMO 3 collaboration is looking for neutrinoless double beta decays to investigate Majorana neutrino mass in the sub-eV region. Construction of the NEMO 3 detector has been completed, and the detector is now operating normally in the Fréjus underground laboratory (Laboratoire de Souterrain de Modane, LSM). The design of the detector and the expected performances are reviewed briefly. The preliminary analysis of the initial runs of ¹⁰⁰Mo double beta decay is presented here.