The present invention relates to a technique for encoding and decoding an image, and a method for encoding the image, according to the present invention, comprises the steps of: performing a prediction with respect to a current block; and transforming a residual which is generated by the prediction, wherein in the step of transforming, a first transform is performed on the residual, and then a second transform is performed on a lower frequency area, and wherein a mode of the second transform can be decided on the basis of a mode of the first transform.