  We consider an expansion of the type-I seesaw mechanism by the inverse of the
3-3 matrix element $1/(M_{R})_{33}$ of the mass matrix of right-handed
neutrinos $M_{R}$. Conditions of such a situation are obtained for $M_{R}$ and
the Dirac mass matrix $m_{D}$.
  In this case, a partial $Z_{2}$ symmetry such as $S m_{D} P_{} = \pm m_{D}
P_{}$ with a projection matrix $P_{} = {\rm diag} ( 1, 1, 0)$ leads to an
approximate $Z_{2}$ symmetry by $S$ for the neutrino mass matrix $m_{\nu}$.
Such a partial $Z_{2}$ symmetry is desirable in the context of unified theories
because it allows hierarchical $m_{D}$ and the large mixing of $m_{\nu}$
simultaneously.
