  There are experimental indications of large flavor mixing between $\nu_{\mu
}$ and $\nu_{\tau}$. In the unification models, in which the massless sector
includes extra particles beyond the standard model, there possibly appear the
mixings between quarks (leptons) and the extra particles. When large mixings
occur, lepton flavor mixings can be quite different from quark flavor mixings.
By taking the string inspired $SU(6) \times SU(2)_R$ model with global flavor
symmetries, we obtain the neutrino flavor mixing $\sin \theta_{23} \simeq
\lambda = \sin \theta_C$ around the unification scale. It can be expected that
due to large Yukawa couplings of neutrinos, the renormalization effect
increases $\sin 2 \theta_{23}$ naturally up to $\sim 1$ at the electroweak
scale. Fermion mass spectra and the CKM matrix of quarks obtained in this paper
are also phenomenologically viable.
