Natural language has expressions which are used for the utterer's emotion, which I call emotionals. Although the analysis of expressing emotions is a kind of psychological research, I will propose a theory of emotion expressing by emotionals by exploiting the theory of belief revision and theory change, which has a long tradition such as the AGM-Theory, the KM-theory, and Dynamic Doxastic Logics (DDL). I will classify emotionals, in particular, which are also belong to sentence adverbials, or modal expressions of English and Japanese, into 13 classes. Then I will provide a dynamic semantics based on the belief revision theory and a coalgebraic theory of belief revision processes, called the theory of system transition systems. As a result, emotions such as surprise, depression, angry, sadness, pleasure, and fear are partially formalized in terms of revision of beliefs desires, intentions, and unconscious beliefs, with plausibility, which is formalized by Friedman.