1. A stereo image processing apparatus comprising: image input unit for inputting, as main images, plural images constituting stereo images; encoding circuitry for encoding stereoscopic display information with respect to the plural main images which have been inputted by the image input unit into an image; and image synthesis unit for synthesizing the plural main images which have been inputted by the image input unit and an encoded image which has been obtained by the encoding circuitry, wherein the image synthesis unit synthesizes padding data as the encoded image indicating whether the plural main images are either stereo images generated by a parallel method or stereo images generated by a crossing method with the plural main images, and further comprising: an aspect ratio adjustment unit applying aspect ratio adjustment control, wherein the image synthesis unit changes size of the padding data so that longitudinal and lateral sizes of a synthetic image have an aspect adjustment ratio adjustment that is determined by the aspect ratio adjustment unit and which is also applied to the image data.