1. An image predictive decoding method for decoding an input coded data by referring to a reference image including an object image comprising a luminance signal indicating a pixel value and a shape signal indicating whether a pixel is located inside or outside the object image, said method comprising: obtaining a substitute pixel value for a pixel located outside the object image, which is determined according to the shape signal, in a subarea of the reference image by using a pixel value of a pixel located inside the object image, which is determined according to the shape signal, in the subarea of the reference image; generating a padded predictive subarea by padding the pixel located outside the object image with the substitute pixel value; decoding the input coded data to obtain a decoded difference data; and adding the decoded difference data and the padded predictive subarea to generate a decoded image.