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, $y_��$, into a triangular form, $y_{\triangle}$. The mixing matrix of light leptons is $V_{KM} S^{\prime *}$, where $V_{KM} \equiv {V_{Le}}^{\dagger} V_{L ��}$ %$V_{Le}$ and $V_{L��}$ diagonalize %the Yukawa matrices of charged leptons, %$y_e y_e^{\dagger}$, and neutrinos, $y_�� y_��^{\dagger}$, %respectively, and $S^{\prime *} \equiv {V_{L��}}^{\dagger} S^*$. 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.

14 pages, 4 figures, PTPTeX