An AI decoding apparatus includes a memory storing instructions and a processor configured to execute the instructions to obtain AI data related to AI down-scaling of an original image and image data generated as a result of encoding a first image, obtain a second image corresponding to the first image by decoding the image data, determine a resolution ratio in a horizontal direction and a resolution ratio in a vertical direction between the original image and the first image, based on the AI data, and obtain, by an up-scaling deep neural network (DNN), a third image in which a resolution in at least one of a horizontal direction and a vertical direction is increased from the second image based on the resolution ratio in the horizontal direction and the resolution ratio in the vertical direction.