In a playback method of a playback device that plays a system stream file including encrypted video information, the system stream file includes a first segment in which is repeated a first data unit that can be decrypted using a first decryption key that the playback device has and a second data unit of the same data size as the first data unit but cannot be decrypted by the first decryption key, and a second segment which can all be decrypted using a second decryption key that differs from the first decryption key. The playback method reads the system stream file, and performs conversion processing to convert the system stream file that has been read out into a post-conversion system stream, in which the second data unit in the first segment in the system stream file that has been read out is replaced with a plurality of continuous invalid packets, and outs the post-conversion system stream.