  It is a longstanding and remarkable problem when and how red S0 galaxies were
formed in clusters of galaxies.
  We here propose that the major mechanism for the S0 creation is galaxy
merging between two spirals with unequal mass.
  Our numerical simulations demonstrate that galaxy merging exhausts a large
amount of interstellar medium of two gas-rich spirals owing to the moderately
enhanced star formation, and subsequently transforms the two into one gas-poor
S0 galaxy with structure and kinematics strikingly similar to the observed
ones.
  This secondary S0 formation with enhanced star formation explains a smaller
fraction of S0 population recently observed in some distant clusters of
galaxies.
  Unequal-mass galaxy mergers thus provide an evolutionary link between a
larger number of blue spirals observed in intermediate redshift clusters and
red S0s prevalent in the present-day ones.
