  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 ($<m_\nu>$) 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 ($\bar{m_\nu}$) of single beta decay. The lower mass
bound for $\bar{m_\nu}$ is given from the present neutrino oscillation data. We
obtain some relations between $<m_\nu>$ and $\bar{m_\nu}$. The constraints on
the Majorana CP violating phases are also given.
