[en] The Fukushima Daiichi Nuclear Power Plant accident caused a release of radionuclides. Radionuclides were deposited on the ground not only in Fukushima prefecture but also in nearby prefectures. Since the accident, measurement of radiation in environment such as air dose rate and deposition density of radionuclides has been performed by many organizations and universities. In particular, Japan Atomic Energy Agency (JAEA) has been performing observations of air dose rate using a car-borne survey system continuously and over wide areas. In our study, using the data measured by JAEA, we estimated effective dose from external exposure in the six prefectures adjacent to Fukushima prefecture. Since car-borne survey was started a few months later after the accident, measured air dose rate in this method is mainly contributed by ¹³⁷Cs and ¹³⁴Cs whose half-lives are relatively long. Therefore, based on air dose rate of ¹³⁷Cs and ¹³⁴Cs and the ratio of deposition density of short-half-life nuclides to that of ¹³⁷Cs and ¹³⁴Cs, we also estimated effective dose contributed from not only ¹³⁷Cs and ¹³⁴Cs but also other short half-life nuclides. We compared the effective dose estimated by the method above with that of UNSCEAR and measured data using personal dosimeters in some areas. (authors)