The six core competencies included in the mathematics curriculum revised in 2015 are problem solving, reasoning, communication, attitude and practice, creativity and convergence, information processing. In particular, the reasoning is very important for students' enhancing much higher mathematical thinking. Based on this competency, this study selected the four elements of investigation and fact guess, justification, the logical performance of mathematical content and process, reflection of reasoning process, And also this study selected the domain of function which is comprised of the content of the coordinate plane, the graph, proportionality in the seventh grade mathematics textbook. By the subject of the ten kinds of textbook, this study examined how the four elements of the reasoning competency were shown in each textbook.