This paper analyzes the Japanese particle “o” used with Japanese intransitive verbs of the “tatakau” type. As a result of the analysis, it was observed that these “o”case noun phrases can be divided into two types (the event type and the duration type), and that the particle “o” marking each type of noun is functionally differentiated. Specifically, the “o” case noun phrases of the event type are connected to intransitive verbs of the “tatakau” type through a transitive relationship, and they grammatically behave like objects. On the other hand, it was found that the “o” marking duration-type nouns serves an adverbial function and can be used freely under certain conditions. From the fact that the particle “o” with two functions is used in intransitive verbs of the “tatakau” type, it can be seen that the particle “o” has a variety of meanings and usages. This study is significant in that, through such an analysis, it enables the discovery of a wider variety of combinations between unergative verbs and the particle “o,” as well as the analysis of those usages.