External data acquired via an input device during display of presentation information on a display device is recorded, and feature amounts are calculated from the external data. An importance level is calculated by integrating the feature amounts for each section during the display period of the presentation information. First headline information of a part of the presentation information corresponding to a section having an importance level which exceeds a predetermined threshold is generated, and is presented to be aligned along the time axis. Previous display contents of the presentation information are searched for in response to a selection operation of the first headline information, and are displayed on the display device. Second headline information of a part of the presentation information corresponding to the section when the selection operation was made is generated, and is presented on the display device.