A writing device constituting an electronic book includes a connector to which electronic papers are detachably connected. The number of connected sheets of electronic paper is recognized, and is output to a host computer. In the host computer, display data is output on the basis of the number of electronic papers, and the display data is written in the electronic papers by a write control unit. When a change of connections of the electronic papers is detected by the detection unit, a part of images displayed on the electronic papers is rewritten depending on the detected connection states of the electronic papers by which optimum images on the electronic papers are displayed even if an arbitrary electronic paper is attached or detached.