The purpose of this paper is to measure the external costs in a land improvement project involving paddy fields in Hokkaido, Japan. We calculated the environmental loads emitted by fossil fuels and construction materials in the construction stage. We also computed the decreasing environmental loads in the cultivation stage because such a land improvement project has the effect of reducing the fossil fuel consumption of agricultural machines. The environmental loads calculated in this paper were converted into external costs using LIME (Life-cycle Impact assessment Method based on Endpoint modeling).