A large area of east Japan was contaminated by radiocesium following a nuclear accident at the Fukushima Daiichi Nuclear Power Station. Following decontamination of the soil, external effective dose conversion factors were calculated by changing the volume of decontamination soil, depth of cover soil, and distance of the evaluation point from the decontamination soil. The decrement of the factors with an increase of the distance was larger for the smaller volume of decontamination soil. The factors decrease exponentially with an increase of the depth of cover soil in all cases. When there was no cover soil, annual external exposure doses for residents at 1 m from the repository site and public entry were over 10 μSv/y, even for the smallest size (2m × 2m × 1m) and 50 percentile value of radiation concentration (700 Bq/kg). When the surface was covered by 30 cm of non-contaminated soil, the annual external exposure doses were less than 10 μSv/y for the largest size (200m × 200m × 10m) and 95 percentile concentration (2500 Bq/kg).