  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 appear in the lepton number violation processes such as
neutrinoless double beta decay, the $\mu^--e^+$ conversion, and the K decay,
$K^-\to\pi^+\mu^-\mu^-.$ Using this graphical representation, we derive the
constraints on the CP violation phases in the lepton sector.
