  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 valid 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.
