The purpose of this study is to formulate the simplified method of estimating the environmental load of consisting component for detached house. In this paper, investigating the actual prefabricated houses in detail, analyzing the approximate 5,000 pieces of materials to convert into CO_2 emission amount, the relationship between CO_2 emission amount and houses' form/scale was examined. It was shown that the proposed formula could estimate the amount of emitted CO_2 easily at practical accuracy by using a total floor area.