A data processing apparatus and method and a computer readable storage medium storing data processing programs realizing the data processing method, capable of preventing transmission of unnecessary redundant data and efficiently utilizing a transmission path. In the data processing method, data is input, the data is compression-encoded, the encoded data is packetized into a first data train on a basis of a first data length, the generated first data train is packetized into a second data train on a basis of a second data length, and the first data length is controlled in accordance with the second data length.