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We estimate dark matter (DM) density for the Universe with a reheating temperature smaller than the mass of DM, assuming DM 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. DMs 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 DM is much larger than that estimated in the literature and can be consistent with that observed when the mass of DM is larger than .