In this study, we collected Japanese nuclear power plant construction cost data from official documents submitted by the electric utilities and conducted a quantitative analysis of the past trends. We found that the unit construction cost of Japanese nuclear power plants rose during the period from 1975 to 1980, when the "improvement and standardization" programs took place, and did not increase or decline significantly after that. We also observed significant economies of scale, even if we take into account interest during construction, as well as the so-called overnight cost. As far as we know, this study is the first attempt to analyze the total history of Japan's nuclear power generation until the Fukushima accident from the cost perspective. The findings could contribute to a better understanding of the economics of nuclear power, as similar studies in the United States and France tend to exhibit different results. The analyses in this study appear to reinforce the reliability of the cost estimation by the Japanese government, which has been used as the numerical basis for the current energy policies in Japan.