  We assume that neutrinoless double beta decay is caused by the exchange of
three light Majorana neutrinos. Under this assumption, we obtain, by the method
of perturbation, the equation representing the isocontour of effective Majorana
mass which is the function of two CP-violating Majorana phases. The equation
representing the isocontour (constraint equation between two Majorana phases)
is expressed analytically by six parameters: two lepton mixing angles, two
kinds of neutrino mass squared differences, lightest neutrino mass scale, and
the effective Majorana mass. We discuss how the constraint equation between two
Majorana phases changes when the lightest neutrino mass scale is varied.
