In recent times, researchers in pedagogy have focused on digital learning logs collected by learning tools such as Learning Management Systems and digital textbook systems. By analyzing and visualizing these, they aim to improve learning and/or teaching methodologies in the future. Using a digital textbook system, it is possible to collect information on which textbook pages were browsed by learners. However, these tools cannot decipher which zones of the textbook were browsed. In order to collect this information, eye-tracker technology would be necessary, but providing each learner with an eye-tracker would be too expensive. To solve this problem, a previous work proposed a method that used masks to detect and conceal each section of the slides in a digital textbook. The learner then clicked the masks one by one to delete them while browsing the contents of the digital textbook. By recording the learner’s clicking operations, the method collected information about the zones browsed by the learner. However, this method was found to cause a decline in learning achievement and system usability as a large number of zones were hidden. Therefore, we propose a grouping method, based on the layout information of the slides, in order to identify the appropriate zones to hide with masks.