For knowledge representation based on ontology and its use, it is desirable to understand phenomena in the target world as precisely and deeply as possible. The ontology should reflect the understanding of them and provide a fundamental framework to manage the behavior of instances adequately. The management of instance model requires identity of things. Contrary to the common understanding, there are several kinds of identity according to the purpose of its use. This paper discusses how many kinds of identity exist and what kind of identity suits to what purpose. Based on the consideration result we suggest four kinds of identity and discuss what situation to be applied.