The purpose of this article is to consider the meaning of authenticity of teaching materials in English language education, and consequently to clarify the viewpoints for developing teachers’ abilities in studying teaching materials. In second language acquisition studies, it is regarded as essential to be exposed to abundant comprehensible input in the target language. With the advancement of ICT and a variety of educational software, second language learners have the great advantage of getting access to a lot of natural English language resources. Under the recent communicative trend, there is a strong belief that teaching materials must be as real as possible; however, there remains certain skepticism about the effectiveness of authentic materials in second language teaching. Furthermore, there is no unified definition of the term. Therefore, this article will review the concepts of authenticity advanced by several applied linguistics researchers and examine efforts to maintain authenticity in textbook materials adaptation processes. By theoretical consideration of material authenticity and practical analysis of the simplifying process from the original to the adapted textbook material in a Japanese junior high school English textbook, this study will try to construct a viewpoint of authenticity necessitated in an English-as-a foreign-language situation like Japan.