1. A data editing apparatus that edits data about a plurality of book sheets of a book, each book sheet belonging to one of a plurality of chapters, to bind the plurality of book sheets by a casing-in operation in such a manner that chapter range images, which indicate respective ranges of the plurality of chapters, are displayed on an edge of the book so that the chapter range images that are adjacent to each other are arranged at different positions from each other in a top-bottom direction, and the data editing apparatus comprising: a marker image edit unit that judges whether a number of pages of each chapter is less than or equal to a setting threshold and edits the data in such a manner that a marker image emphasizing the chapter range image is adjacent to the chapter range image indicating the range of a chapter having book sheets the number of which is less than or equal to the setting threshold.