  We estimate dark matter density for the Universe with a reheating temperature
smaller than the mass of dark matter, assuming dark matter to be a weakly
interacting massive particle. During the reheating process, an inflaton decays
and releases high-energy particles, which are scattered inelastically by the
thermal plasma and emit many particles. Dark matters are produced through these
inelastic scattering processes and pair creation processes by high-energy
particles. We properly take account of the Landau-Pomeranchuk-Migdal effect on
inelastic processes and show that the resultant energy density of dark matter
is much larger than that estimated in the literature and can be consistent with
that observed when the mass of dark matter is larger than $O(100)$ GeV.
