  We discuss the constraints of lepton mixing angles from lepton number
violating processes such as neutrinoless double beta decay, (\mu^-)-(e^+)
conversion and K decay, $K^- \to \pi^+\mu^-\mu^-$ which are allowed only if
neutrinos are Majorana particles. The rates of these processes are proportional
to the averaged neutrino mass defined by $<m_{\nu} >_{a b}\equiv
|\sum_{j=1}^{3}U_{a j} U_{b j}m_j|$ in the absence of right-handed weak
coupling. Here $a, b (j)$ are flavour(mass) eigen states and $U_{a j}$ is the
left-handed lepton mixing matrix. We obtain the consistency conditions which
are satisfied irrelevant to the concrete values of CP violation phases (three
phases in Majorana neutrinos). These conditions constrain the lepton mixing
angles, neutrino masses $m_i$ and (< m_{\nu} >_{a b}). By using these
constraints we obtain the limits on the averaged neutrino masses for
(\mu^-)-(e^+) conversion and K decay, $K^- \to \pi^+\mu^-\mu^-$.
