Thermal inflation is the most plausible mechanism that solves the cosmological moduli problem naturally. We discuss the relic abundance of a superheavy particle X in the presence of thermal inflation, assuming that its lifetime is longer than the age of the universe, and show that the long-lived particle X of mass (Formula presented) GeV may form a part of the dark matter in the present universe in a wide region of parameter space of the thermal inflation model. The superheavy dark matter of mass (Formula presented) GeV may be interesting in particular, since its decay may account for the observed ultrahigh-energy cosmic rays if the lifetime of the X particle is sufficiently long. © 1999 The American Physical Society.