In image resizing process, preserving structure on an image is important to produce plausible results. We proposed seam merging, a seam-carving-based image resizing method, and a new energy criterion for structure preservation. Seam merging reduces the size of an image by merging a two-pixel-width seam into a one-pixel width seam. To find the optimal seam that minimizes structural distortion after merging, we introduce an energy function which has a large value when the merging breaks the structure. We further show the way to find the optimal seam using dynamic programming. Experimental results demonstrate that our proposal algorithm can produce fewer artifacts and successfully preserve structure.