[en] Questions whether the neutrino is a Majorana particle or a Dirac particle and whether massive or massless are quite important in view of the grand unification of interactions and the galactic dark matter. The most sensitive probe to study the massive Majorana neutrino is a neutrinoless double beta decay (Oν β β). The double beta decays can be seen when a given isotope0is unstable energetically in the isobar chain but is stable against the single β^- and β⁺ decays. There are 35 β^-β^- and 40 β⁺β⁺ isotopes. The double beta decay experiments for the last decade have been mainly focussed on ⁷⁶Ge, because the ⁷⁶Geβ β isotope is contained in a natural Ge detector with 7.8% abundance. Thus the β β events are almost fully detected by the Ge detector. The β β counter experiments for nuclei other than ⁷⁶Ge are aimed to observe clearly the 2ν mode, which should occur necessarily because of the absence of violation of any conservation laws and symmetries. The β β sources with large nuclear sensitivity are chosen. Since the ⁸²Se and ¹⁰⁰Mo isotopes have large Q_ββ values of about 3 MeV, they are the best candidates. In order to explore a possible finite Majorana neutrino mass in the range of 0.1-1 eV and a finite right handed weak current in the range around 10^-6, new large scale detector systems should satisfy some requirementrs for large aystem sensitivity and mode selectivity. The ¹⁰⁰Mo is one of the best candidates to serve for the search for Oν β β decays. (N.K.)