This invention uses a storage device more effectively than ever before to store image data in a predetermined storage device for future security chasing. A determination unit (203) determines whether related image data which can be considered substantially identical to image data acquired by an acquisition unit (210) is stored in a predetermined storage device (202) for future chasing. If related image data is stored, link information to the related image data is written instead of writing the acquired image data to the storage device (202). On the other hand, no related image data is stored, the acquired image data is written to the storage device (202).