Air dose rates were measured inside a car with an ion chamber detector across Fukushima Prefecture from March 19 to 22, 2011. The dose rate along the Tohoku Expressway ranged from 2 to 6 µSv/h. Relatively high values were obtained around Nihonmatsu City. The dose rate dipped to an exceptionally low value inside a tunnel to what was probably the normal background value before the Fukushima nuclear accident. The dose rate in Fukushima City ranged from 2 to 9 µSv/h. High values were obtained in the center of the city. The dose rates along the Banetsu Expressway and in Samegawa Village located in the Abukuma highlands were very low at 1.5–3 and 1 µSv/h, respectively. During our stay in Fukushima, external and internal exposure was evaluated at 0.1 and 0.0327 mSv, respectively. A commercially available GM-tube detector calibrated with a 137Cs standard source was used to determine the status of environmental radioactive contamination caused by the Fukushima nuclear accident. The calibration was achieved with a quadratic correction function for dose rates up to 4 µSv/h. The calibrated GM-tube detector was used for measuring air dose rates and ground surface contamination in East Japan. Measurements were taken inside a car and a bullet train as well as outdoors from March to August 2011. The dose rates measured inside a vehicle along the Tohoku Expressway (in May) and in the Tohoku Shinkansen (in August) were as much as 1.0 and 0.25 µSv/h, respectively. The measurement results agreed well with the official data obtained using a NaI scintillation counter, demonstrating that a calibrated GM-tube detector can be an effective tool for measuring radioactivity.