To provide means for editing a content data stream quickly. A data processor according to the present invention can edit a content that has been written on a medium. The content is stored in a file as a data stream including multiple packets of the same packet size. And a file system for accessing data on a cluster size basis has been established on the medium. The data processor includes: a receiving section, which receives an instruction on a partial deletion range including the beginning of the file; and a media control section for deleting data, the size of which is an integral number of times as large as the least common multiple of the cluster size and the packet size, from the beginning of the file within the partial deletion range.