A method is provided for detecting an error in an image data stream in a device environment where digital image data is reproduced, especially, in a wired/wireless network such as a personal portable device. The method comprises determining whether or not an error is present in an input image data stream; determining a similarity between patterns of transform coefficients that are generated in the course of decoding the input image data stream; and detecting the starting position of the error based on the similarity determination. Accordingly, a decoder can independently detect the position of the error accurately, and the quality of an image which is replaced and restored using error concealment can be improved.