Radon is a natural, inert, invisible, odorless and chemically inactive radioactive gas emitted from the earth. It is produced by the decay of uranium ore, such as radium, actinium, or thorium.Because inhaling radon and its radioactive decay products causes irradiation of lung tissue, prolonged exposure to high concentrations of radon significantly increases the risk of developing cancer. Various types of equipment and components have been proposed to date for radon detection.In this paper, in order to investigate the performance of the ion chamber-type radon counter : RD200, the radon measurement system : RAD7 was used. Through some experimental studies, we found that the radon data of two RD200s tracked those of RAD7 very closely and RD200 had some advantages over PIN photodiode-type radon counter : Siren PRO3 for excellent sensitivity and prompt display of the radon concentration. Keywords: radon, radon counter, ion chamber-type, RAD7, RD200, radon concentration