This study was originated from a question about the government policy which is to fully replace the current paper textbooks with technically-unstable digital textbooks. The discussions on digital textbooks are partially exaggerated in the aspects of industry, economy and education, and also tend to bring a unrealistic fantasy based on the exaggerated effect. In the results of analyzing the advanced studies on digital textbooks, many studies focused on measuring the effectiveness of academic achievement though there were no common study results finding that the academic achievement level of a digital textbook-use group is higher than the one of a paper textbook-use group. Also there were lots of concerns about negative influence of digital textbooks on students' emotional/social/physical development as students' and teachers' satisfactions with it were not even that high. In the results of comprehensively analyzing the strengths and weaknesses of both paper and digital textbook, we found that both types of textbook can largely supplement each other's weaknesses. In order to prepare measures to aim for the complementary and developmental directions of both types of textbook, this study suggested 5 developmental phases from paper textbooks to digital textbooks. It is such a progressive development phases including the stages in which paper textbook fully performs, the functions of paper textbook is a little decreased, but mainly used when supplemented with digital textbook, both paper and digital textbook are mutually used, digital textbook is mainly used when supplemented by paper textbook, and finally, digital textbook fully performs its roles. The transition into digital textbook should be dealt carefully. Taking strengths and discarding weaknesses of each textbook with continuous development of technology, the core of the transition into digital textbook is the matter of fully using the strengths of each textbook.