Disclosed is a conversion apparatus including a decoding unit that decodes input encoded data into image data, and a size conversion unit that converts size of the image data based on a size conversion request received from outside and that expands/contracts, or splits/synthesizes motion vector or prediction mode of encoding parameters, received from the decoding unit, based upon a size conversion request, and an encoding unit that encodes image data after size conversion using the prediction mode or motion vector converted and transmits the resulting encoded image data via a transmission buffer to outside.