  It is shown how to obtain the analytical expression for the effective mixing
angle in matter using the formalism which was developed by Kimura, Takamura and
Yokomakura. If the baseline of the neutrino path is long enough so that
averaging over rapid oscillations is a good approximation, then with the help
of Landau's method, the nonadiabatic contribution to the oscillation
probability can be expressed analytically by this formalism. We give two
examples in which the present method becomes useful.
