1. An encoding device comprising: a memory; and a processor coupled to the memory, the processor configured to: specify a boundary block from among a plurality of image blocks, the plurality of image blocks being generated by dividing a coupled frame image, the coupled frame image being one of a plurality of coupled images included in a video data, each of the plurality of coupled images being generated by coupling a first image and a second image together so that a right side of the first image comes in contact with a left side of the second image, the boundary block including both a part of the right side of the first image and a part of the left side of the second image, the first image being an image for a left eye, the second image being an image for a right eye; determine first correlation degree and second correlation degree, the first correlation degree being correlation degree between a part of the first image included in the boundary block and an image block included in another coupled frame image, the second correlation degree being correlation degree between a part of the second image included in the boundary block and the image block included in the another coupled frame image, the image block included in the another coupled frame image being generated by dividing the another coupled frame image included in the plurality of coupled images; generate a substitution block by substituting the part of the first image with a substitution image generated based on the part of the second image when the first correlation degree is lower than the second correlation degree, or by substituting the part of the second image with a substitution image generated based on the part of the first image when the second correlation degree is lower than the first correlation degree; and perform an inter-image prediction encoding for the image block included in the another coupled frame image by using the substitution block.