This paper aims at analyzing several cases of investor-State dispute settlement (ISDS) which relate to the feed-in tariff (FIT) of the renewable energy sector, for the purpose of extracting the legal issues involved in them. On the basis of this analysis, it will bring some implications toward Japan, both from the investors' viewpoint and the host-State's perspective. First, in European and North American countries, there have been many cases in which foreign investors (claimants) submitted disputes, against the host-States, before the ISDS concerning the operation and abolishment of the FIT system. In particular, the main topic is to allege a violation of the fair and equitable treatment (FET) obligation stipulated in the applicable international investment agreement. Second, in cases against Spain, the Tribunal either admitted a breach of FET (Eiser case) or did not (Charanne case and Isolux case). The same applies in the cases against Canada. These situations require us to analyze the reason why there has been a difference of conclusions. Third, on the basis of the above analysis, it will become possible to evaluate the modified FIT law of Japan (2016) and present some implications about it.