A motion vector detection circuit detects the motion vector for each macro-block between an odd field and an even field. An encoding system decision circuit decides the type of the encoding system, that is if the encoding system is a field-based encoding system or a frame-based encoding system, based on a median of a motion vector. A control circuit controls gates and changeover switches, in accordance with the encoding system type as decided by the decision system, for generating a field-based reference picture or a frame-based reference picture from buffer memories. The circuitry from an additive node to a VLC circuit finds difference data between the reference picture and the picture to be encoded, while transforming the difference data by discrete cosine transform and variable length encoding the transformed data. The VLC circuit sets the encoding system type as a flag in a header of a predetermined hierarchical layer of a bit stream. By the above operation, any interlaced picture may be encoded efficiently, whether the picture includes little motion or abundant motion or includes both little motion and abundant motion in combination. A picture data decoding device detects the flag and executes decoding by changing over field-based decoding to frame-based decoding or vice versa depending on the flag for reproducing the picture data.