Energy crops are not presently major energy resources as energy crops are more expensive than fossil fuels at present. However, energy crops may become important energy resources in the future. In this study, the authors discuss the availability of energy crops in Japan. The supply potential of energy crops produced on unused arable land is estimated at 0.12 EJ yr(-1) and that of secondary crops for bioenergy is estimated at 0.12 EJ yr(-1) in Japan. However, it is difficult to utilize the supply potential considering the low food-self-sufficiency ratio and the high costs of crops in Japan. In addition, the authors analyze the supply potential of energy crops produced on surplus arable land in Japan in cases of biomass productivity increment. The supply potential of energy crops is formulated into 0.12A (EJ yr(-1)), where A means the index of productivity increment (A = 1.0 at present). On the other hand, in the case of every crop productivity increment, the supply potential of energy crops is formulated into 1.44A-1.32 (EJ yr(-1)). When it is assumed that the ratio is 2.0, the supply potential in the latter case is 1.44 EJ yr(-1), which is equivalent to about 7% of the total primary energy supply in Japan. When it is assumed that the ratio is 2.0 in the latter case in the world, the supply potential of energy crops is 435 EJ yr(-1), which exceeds the total primary energy supply in the world. It is difficult to improve the productivity of every crop. However, if the improvement is realized, energy crops will become one of the major energy resources in Japan and in the world. (c) 2005 Elsevier Ltd. All rights reserved.