  It has passed 20 years after we proposed $\mu-\tau$ symmetry in light
neutrino mass matrix. This model is simple but reproduced the characterestic
properties of lepton sector. After that, during the experimental developments,
there have appeared so many extensions but most of those phenomenological
models are lacking systematic outlooks towards more fundamental theories. In
this paper, we try to consider rather systematic model extensions and
application to GUT model.
