  We investigate when and how a dusty starburst galaxy merger can be heavily
obscured by dust and consequently becomes an ultra-luminous infrared galaxy
(ULIRG), based on numerical simulations of chemodynamical and photometric
evolution of dusty gas-rich major galaxy mergers.
  We found that a major galaxy merger is more likely to become an ULIRG
preferentially in the merger late phase, when the two disks become very close
and the very high-density dusty gas can obscure heavily the central secondary
massive starburst.
  We furthermore show how the optical and near-infrared morphology of a
simulated ULIRG at intermediate (z=0.4) and high redshift (z=1 - 2) can be
observed by the Hubble Space Telescope (HST) in order to present a plausible
explanation for the origin of some host galaxies of the faint SCUBA sources
recently observed by Smail et al. (1998).
  The results of our numerical simulations imply that some SCUBA sources with
apparently faint and compact HST morphology can be higher redshift
dust-enshrouded starburst mergers with their outer low surface brightness tidal
features hardly detectable in the present optics of the HST.
