A high-temperature scanning-tunneling-microscope (STM)/low-energy ion-gun combined system has been developed in order to clarify the microscopic aspects of annealing processes of ion-irradiated Si surfaces. This system enables us to perform atom-resolved high-temperature STM observation and ion beam irradiation simultaneously in ultrahigh vacuum conditions. Taking great advantage of this system, we have successfully obtained sequential STM images of high-temperature Si(111) surfaces irradiated with 3 keV Ar+ single ions. The STM results have shown that the surface defects induced by single ion irradiation show various changes in size and shape, which is considered to result from diffusion of vacancies and interstitial atoms in the substrates, diffusion of atoms on the surface, and from an anisotropic character of the surface atomic arrangement.