Informativity means how the information for acceptor is fresh and unpredictable. It depends on an ability of translator. It can be maintained or not. As for idiomatic expression, it contains higher informativity than typical function word or general content word. Because ambiguity and uncompositionality of idiomatic expression make the process of analyzing semantic information very difficult. Also, interest of reader is induced by the appearance of idiomatic expression in text. We use Korean-English parallel corpus for looking into the phase of informativity change by idiomatic expression. As a result, idiomatic expression is translated to word or phrase on the aspect of linguistic form. It is omitted in the translated text occasionally. It is the most desirable to translate the idiomatic expression of source language to that of target language. But translator is apt to change the informativity involuntarily in general. Implement of the equivalent of meaning is not in accord with maintenance of informativity. Lack of understanding of the original text can be one of the causes by which translator transfers insufficient information to readers. Compilation of a bilingual dictionary containing sufficient information about meaning, usage, and actual examples will help translators to maintain the informativity of idiomatic expression. (The National Institute of the Korean Language)