The mattress for relieving pressure ulcers includes a first sponge layer, an air mattress layer over the first sponge layer, an intermediate layer over the air mattress layer, a second sponge layer over the intermediate layer, and a paper sheet layer over the second sponge layer. A bottom surface of the small impact absorbing member is divided into two portions, a rear half portion located closer to the side airbag and a front half portion located farther away from the side airbag. The identified segments of content are duplicated and then encrypted using a first encryption method associated with a first DRM to produce first encrypted segments. On the basis of a difference value computed by the DLLs 40, the path timing is corrected, thereby producing a demodulation timing. Therefore, it is possible to show a suitable eye-target according to a symptom of a patient, whereby it is possible to conduct training efficiently.