An image data post-processing method for reducing quantization effect induced when image data compressed based on a block is decoded, and an apparatus therefor. The image data post-processing method includes the steps of: (a) detecting semaphore representing whether or not post-processing is required, using distribution of inverse quantization coefficients of inverse-quantized image data and a motion vector representing the difference between the blocks of a previous video object plane (VOP) and blocks of a current VOP; and (b) filtering the decoded image data corresponding to the semaphore by a predetermined method, if it is determined by checking the detected semaphore that post-processing is required. Therefore, the quantization effect can be reduced by using the semaphore and an adaptive filter, and the amount of computation for filtering is also reduced. Also, the complexity of the hardware is reduced by a parallel process without multiplication and division.