  We consider the formation of photon spectrum at the photosphere of
ultrarelativistically expanding outflow. We use the Fokker-Planck approximation
to the Boltzmann equation, and obtain the generalized Kompaneets equation which
takes into account anisotropic distribution of photons developed near the
photosphere. This equation is solved numerically for relativistic steady wind
and the observed spectrum is found in agreement with previous studies. We also
study the photospheric emission for different temperature dependences on radius
in such outflows. In particular, we found that for $T\propto r^{-2}$ the Band
low energy photon index of the observed spectrum is $\alpha\simeq -1$, as
typically observed in Gamma Ray Bursts.
