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.
We illustrate the graphical method that gives the constraints on the parameters appearing in the neutrino oscillation experiments and the neutrinoless double beta decay. This method is applicable in three and four generations. Though this method is v
alid for more general case, we examine explicitly the cases in which the CP violating factors take $pm 1$ or $pm i$ in the neutrinoless double beta decay for illustrative clearance. We also discuss some mass matrix models which lead to the above CP violating factors.
We examine the constraints on the MNS lepton mixing matrix from the present and future experimental data of the neutrino oscillation and lepton number violation processes. We introduce a graphical representation of the CP violation phases which appea
r in the lepton number violation processes such as neutrinoless double beta decay, the $mu^--e^+$ conversion, and the K decay, $K^-topi^+mu^-mu^-.$ Using this graphical representation, we derive the constraints on the CP violation phases in the lepton sector.
