  We investigate roles of the bulk comptonization process in the propagation of
thermal photons emitted at the shock breakout of a supernova explosion. We use
a hydrodynamical model based on a self-similar solution for the shock breakout.
The propagation of photons is treated by a Monte-Carlo technique. Results of
the simulations successfully reproduce the power-law spectrum observed by X-ray
observations for XRF 080109/SN 2008D, if a mildly relativistic shock velocity
is assumed. Effects of some radiative processes, photoionization, radiative
recombination, and free-free absorption on the propagation of emitted photons
are also investigated. It is found that none of them hardly changes the
spectrum regarding the progenitor stars of type Ib or Ic supernovae. Light
curves calculated under the assumption of a spherical explosion indicate that
the progenitor radius is required to be $/sim 10^{13}$ cm.
