The purpose of this paper was to develop a framework for the “author concept” of expository texts from a perspective of “knowledge”. The developed framework has the following four features: (1) the author and reader are scientists; (2) the author belongs to the academic world as a producer of knowledge and he/she explains it to the reader belonging to the life world; (3) the author explains this to the reader belonging to the life world as a mediator of the academic world and the life world; and (4) the author and reader belong to the life world in the life world. I used this framework to examine expository texts on natural science used by third and fourth grade elementary school students.