In an image decoder, anticipated battery energy to be consumed is obtained from consumed battery energy when motion picture data is played for a predetermined time. In case the anticipated battery energy to be consumed is more than a current remaining battery energy, a playing quality is deteriorated to play the motion picture data to the last. Further, the playing quality of the motion picture data is dynamically controlled on the basis of the number of frames that can be displayed during a unit time. Thus, in case there is enough power for processing in a CPU, the playing quality of the motion picture data is improved. In case there is not enough power for processing in the CPU, the playing quality of the motion picture data is deteriorated to complete a decoding process within the unit time. Thus, the playing quality of the motion picture data is dynamically controlled in accordance with the CPU frequency or the remaining battery time.