This review summarizes anomalous variations in radon concentration in Fukushima, Miyagi, and Tochigi Prefectures prior to the 2011 Tohoku-Oki Earthquake. Atmospheric radon concentrations in Fukushima, Hokkaido (Sapporo), and Wakayama Prefectures were analyzed based on at least five years of raw data, whereas the data periods obtained in at Miyagi (around approximately four years of data) and Tochigi (around approximately three years of data) Prefectures before the 2011 Tohoku-Oki Earthquake were shorter than five years. The data were fitted using sinusoidal regression to describe seasonal variations in atmospheric radon concentration. In 72% of prefectures, including the Miyagi and Tochigi Prefectures, the anomalous data extracted from the normal pattern of annual radon variation could be used to identify earthquake activity. We obtain anomalous results that the radon concentrations were simultaneously reduced in the Fukushima, Miyagi, and Tochigi Prefectures before the 2011 Tohoku-Oki Earthquake by analyzing the variations in radon concentration based on the normal seasonal variations in atmospheric radon concentration approximated by the sinusoidal regression curves.