We examine the constraints on the MNS lepton mixing matrix =66rom the present and future experimental data of the neutrino oscillation, tritium beta decay, and neutrinoless double beta decay for Majorana neutrinos. We show that the small mixing angle solutions for solar neutrino problem are disfavored for small averaged mass ($<m_ u>$) of neutrinoless double beta decay ($leq 0.01$ eV) in the inverse neutrino mass hierarchy scenario. This is the case even in the normal mass hierarchy scenario except for very restrictive value of the averaged neutrino mass ($bar{m_ u}$) of single beta decay. The lower mass bound for $bar{m_ u}$ is given from the present neutrino oscillation data. We obtain some relations between $<m_ u>$ and $bar{m_ u}$. The constraints on the Majorana CP violating phases are also given.
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