Radiation monitoring is conducted in accordance with the Comprehensive Radiation Monitoring Plan (CRMP), and includes measuring radiation levels in the soil, water, atmosphere, as well as in wildlife, plant life, food products, and at farms. The CRMP was devised by the central government and Fukushima Prefecture after the accident at the nuclear power plant. It was published on August 2, 2011 and last revised on April 1, 2020.
As of September 2020, ambient dose rates are being monitored at 4,383 locations throughout Japan, including 3,626 locations within Fukushima Prefecture. Dose rates are being measured by transportable monitoring posts and real-time dose rate meters, and readings are published online in real-time, and updated every 10 minutes. You can check the readings at the Monitoring Information of Environmental Radioactivity website, operated by the Nuclear Regulation Authority (NRA).
For information on aerial radiation monitoring, see the response to the following FAQ: Has most of Japan been contaminated with radiation?
Nuclear Regulation Authority, Japan: Monitoring information of environmental radioactivity level -Monitoring Plans Nuclear Regulation Authority, Japan: Monitoring information of environmental radioactivity level -Monitoring results Nuclear Regulation Authority, Japan: Monitoring information -Monitoring results (Japanese)