Recently the fuel ethanol production from crops has attracted much attention not only in the US, EU and Brazil, but also in Japan because the bio-ethanol is perceived as being "carbon neutral" and useful as a countermeasure against global warming. In fact, the Japanese government has declared that the production of 780 thousand kiloliters of bio-ethanol which is equivalent to 500 thousand kiloliters of oil and could reduce CO2 procduction by 1.2 million tons, should be a goal for 2010. However, there are many problemes in bio-ethanol production, pointed out from various points of view. First of all, the utility of bio-ethanol production as a countermeasure against global warming should be examined carefully and objectively. In this study an assessment on the validity of bio-ethanol production and use was tried as to quantitatively estimate the effectiveness of bio-ethanol.