Provided are an image decoding method, according to an embodiment, capable of obtaining a residual signal of an image from a bitstream, changing a ratio of sizes of adjacent first blocks among a plurality of first blocks including the residual signal based on block ratio information obtained from the bitstream, and decoding the residual signal based on the changed ratio, wherein the block ratio information indicates one of predetermined ratios to which the ratio of the sizes of the adjacent first blocks is changeable; and an image decoding device capable of performing the image decoding method. Provided are also an image encoding method, according to an embodiment, capable of being performed in an opposite or similar way to the image decoding method; and an image encoding device capable of performing the image encoding method.