1. A three-dimensional image data generating method that generates a combined image while arranging a left eye image and a right eye image, the method comprising: a combined image generating step in which the combined image includes a left eye image, right eye image, and a buffer region between the left eye image and the right eye image is generated while the left eye image and the right eye image are arranged; and an image data compressing step in which a picture signal, which includes the combined image, is compressed with lossy compression, wherein the combined image generating step includes an image luminance changing step in which the buffer region is formed by making luminance of edges of the left eye image and the right eye image that adjoin each other lower toward the edges.