English text readability employing numerical calculation has been widely accepted by U.S. reading pedagogy, but it has not been accepted in Japan. This paper, after discussion of Japan's rejection of readability, introduces how text readability has been assessed and its assessment practically utilized in American education, and proposes the active use of readability formulas in English language teaching in Japan. Emphasis in ELT textbook compilation in Japan's secondary education has been on strict observance of the Course of Study, which stipulates the number and the kind of vocabulary and sentence structures. Besides, the text-centered attitude in the traditional grammar translation method still prevailing in Japan's educational environment does not put so much emphasis on proper matching between the students (readers) and the textbook (text). Both of the reasons above have turned the teachers' eyes away from the text readability. Thus, research on reading done without objective assessment of text readability reduces its validity for wider application of the results. Readability, which, originated in the 1920s, was first elaborated for wide use by Dale & Chall, based on the assessment of the difficulty of the vocabulary and the complexity of the sentences used in the text. Cognitive psychology and linguistics criticized the formulas in terms of that they did not rightly value the abstract and symbolic expressions and coherence in the sentences. However, the difficulty of numerical assessment of these factors and high correlation between the existing readability scores and actual readability lead to the revised methods of readability assessment. As for actual assessment in the classroom, the Cloze Readability Score is introduced as a practical way to assess the matching between the text and the proposed readers and its implication in reading classes. Two formulas named Flesch Reading Ease and Fog Index are demonstrated with the extracts from junior high school and university textbooks, best-seller and children's books, and TIME and Newsweek. While the readability of best-seller and children's books stays within the 'standard' level, and TIME and Newsweek at 'fairly difficult', college textbooks used in Japan's reading classes range from 'fairly difficult' to 'extremely difficult'. Thus, reading textbooks employed college English classes are sometimes too difficult for Japanese students even compared with those used for the native English speaking counterparts. This unmatching readability of the text is another reason for tangue-tied Japanese students. The author proposes the practical use of these readability assessment tools and employment of English textbooks appropriate for the Japanese students.