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This paper aims at evaluating technologies to realise a sustainable energy system within the constraints of resources and environment. For this purpose, we first developed a database to estimate life cycle efficiencies and greenhouse gas emissions for various energy technologies. Then, on the basis of this life cycle assessment, we quantitatively define the concept of sustainability limitations on resource depletion and environmental emissions. From this concept, the present energy system of the world is judged to be unsustainable from both resource and environmental viewpoints. Then, various mitigation measures such as disposal of CO2 and solar power generation are evaluated utilising the concept. The evaluated result showed how sustainability influences cost–effectiveness of the measures. At the same time we clarified the key points to realise a sustainable energy scenario in ultra–long term.