For challenging a global issue of quality improvement in education, the quality of textbooks should be focused on. This study suggests a view point for textbook quality as the conductivity of information through related actors: a curriculum developer, a textbook writer, a teacher and a student. Then four main roles of a textbook are clarified by simplifying the relationships among the actors, strengthened by literature reviews about the history of textbooks. One of the roles is being a learning material for teachers to convey what to learn, and it is rather subsidiary as a role. Another role is to be a learning material for pupils/students to convey what to learn. It is considered to be the most historical role. The third role is being a teaching material for teachers to convey how to teach, which is rather new as a role. The last role is as a learning material for pupils/students to convey how to learn, and it is a newer role. To conclude, the suggested four roles of a textbook are not functioned until the information is completely passed through all the walls called actors. If any intended roles are not functioned, there is an inhibitory factor in the information conductivity. In addition, identifying the location of the factor such as “between which actor and which actor" helps identifying a problem to solve for textbook development in the improvement of educational quality in developing countries.