Anomalous absorption of an intense short laser pulse in overdense
plasmas is analyzed with a stochastic theory. A diffusion equation
describing a time evolution of the electron distribution function
is derived. From the equation it is shown that the electron
distribution function becomes anisotropic in the momentum space,
which gives rise to the absorption of the energy. The diffusion
is not dominant in the pz direction,
which is longitudinal to the vacuum-plasma boundary, rather it is
dominant in the px direction, which is
transverse to the boundary. However, the diffusion in
pz enhances the absorption. Analytical
expressions of the absorption coefficient and skin depth are obtained
for the anomalous skin effect regime (ω02c2 <<
ωp2ve2),
which evolves in time as the electron distribution becomes
anisotropic. The asymptotic value of the absorption coefficient is
proportional to [square root]I. The temperature and density
dependence of the absorption coefficient is also discussed.