It is the“ mitigation hierarchy” that achieve balancing human activities, including real estate development, and nature conservation. Mitigation hierarchy is the rule which requires development proponents to firstly avoid, secondly minimize, and, as a last resort, to compensate for environmental damage. “Compensatory mitigation” is called “biodiversity offset” as well and is the way to achieve no net loss of the same type of ecosystems which are to be destroyed by proposed development projects. Many countries, including most developed countries, have already introduced this rule but we have just started to discuss them in Japan. In this paper, a compensatory mitigation project stemming from high grade residential area development in the US, in which I participated 23 years ago, was explained and then discussed in order to understand the indispensability of　“mitigation sequence” in Japan.