The Osaka Basin in western Japan is a typical sedimentary basin composed of thick, soft sediments with a maximum thickness of 3 km in the sea zone and of 1.8 km in the plain zone. This geologic setting results in the prevalence of long‐period (above 1 s) strong ground motions with long‐lasting durations of several minutes inside the basin. To mitigate earthquake damage to long natural‐period structures caused by future large (M 8 or larger) interplate earthquakes, Nankai and Tonankai earthquakes, we develop a map of the predominant period of the Osaka Plain by using array earthquake‐motion recordings and a 3D basin model. This map is useful for planning high‐rise buildings and detection of risky existing high‐rise buildings that may be resonant with the long‐period strong ground motions. Using the map, we actually detect several risky high‐rise buildings.