  We study the neutrino mixing matrix (the MNS matrix) in the seesaw model. By
assuming a large mass hierarchy for the heavy right-handed Majorana mass, we
show that, in the diagonal Majorana base, the MNS matrix is determined by a
unitary matrix, $S$, which transforms the neutrino Yukawa matrix, $y_{\nu}$,
into a triangular form, $y_{\triangle}$. The mixing matrix of light leptons is
$V_{KM} S^{\prime *}$, where $V_{KM} \equiv {V_{Le}}^{\dagger} V_{L \nu}$
%$V_{Le}$ and $V_{L\nu}$ diagonalize %the Yukawa matrices of charged leptons,
%$y_e y_e^{\dagger}$, and neutrinos, $y_{\nu} y_{\nu}^{\dagger}$,
%respectively, and $S^{\prime *} \equiv {V_{L\nu}}^{\dagger} S^*$. Large mixing
may occur without fine tuning of the matrix elements of $y_{\triangle}$ even if
the usual KM-like matrix $V_{KM}$ is given by $V_{KM} =1$. This large mixing
naturally may satisfy the experimental lower bound of the mixing implied by the
atmospheric neutrino oscillation.
