It is important for future NLP systems to formulate the semantic equivalence (and more generally, the semantic similarity) of natural language expressions. In particular, paraphrasing, full text information retrieval, example-based MT and document compression technology require the effective equivalence criterion for linguistic expressions. In this paper, first, we discuss the meaning of Japanese sentence-final modality expressions (ME) and second, present equivalence rules for paraphrasing a string of MEs while preserving its meaning.