  A Landau-Lifshitz-Gilbert type equation has been derived by using s-d model
in which the s-electron system is regarded as an environment coupled weakly
with the localized spins. Based on the irreducible linear response theory, we
show that the relaxation function of the s-electron spin leads to the Gilbert
type damping term which corresponds to the retarded resistance function in the
generalized Langevin equation. The Ohmic form of the Gilbert term stems from
the fact that the imaginary part of the response function (spin susceptibility)
of the itinerant electron system is proportional to the frequency (omega) in
the low omega region. It is confirmed that the Caldeira-Leggett theory based on
the path-integral approach gives the same result.
