In the framework of Systemic Functional Linguistics (SFL) founded by M. A. K. Halliday, copular sentences are classified as Relational Process in the category of Transitivity. This paper considers similarities and differences between English copular sentences and their Japanese counterparts in terms of Transitivity. Throughout our discussion, we show that the so-called Japanese copula 'da', its written form 'dearu' and its honorific form 'desu', are actually the indicators of Modality, such as in confirmed declarative force and also plus / minus deferential implications. It should be noted that, in Japanese, adjectival predicates are never accompanied by the copula 'da' or its written form 'dearu', and that they are usually used without the accompaniment of the honorific form 'desu'. This means that, in Japanese, adjectivalpredicate sentences are very similar to verbal-predicate sentences in the perspective of Transitivity, while so-called keiyo-doshi (adjectival-verbal, or na-adjectival)-predicate sentences are in turn near to nominal-predicate sentences.