A data processor acquires video data and audio data and, after processing them appropriately, stores them on an optical disc and/or an HDD (300). When displaying an index of the images representing the acquired video data, thumbnail images are generated by means of a decoder (480) and an AV processor (460) on the basis of the data of the data system recorded along with the video data. The thumbnail images of the data system that can be used for displaying images are displayed as index, while the thumbnail images of any data systems other than the above data system are replaced by respective invalid images to tell that they cannot be displayed. Thus, an index can be displayed with a simple arrangement if data of different data systems are recorded.