An MPEG downdecoder is to be provided which eliminates dephasing of pixels during motion compensation to prevent deterioration of the picture quality ascribable to motion compensation. MPEG data of a high resolution picture are processed by decimating IDCT devices 14, 15 with 4×4 decimating IDCT to decode data of standard resolution picture data. In the case of the field motion prediction mode, a motion compensation device 18 interpolates respective pixels of a macro-block of reference picture data stored in a frame memory 17 to generate a macro-block constituted by pixels of ¼ pixel precision. In the case of the frame motion prediction mode, a motion compensation device 19 interpolates respective pixels of macro-block of reference picture data stored in the frame memory 17 to generate a macro-block constituted by pixels of ¼ pixel precision.