1. A video decoding method using inter prediction, the method comprising: obtaining motion information and residual data from encoded data; generating a predicted block by performing the inter prediction based on the obtained motion information; identifying one or more conditions for enabling an inverse quantization and an inverse transform; skipping both of the inverse quantization and the inverse transform for the obtained residual data to thereby reconstruct a residual block directly from the obtained residual data, if the one or more identified conditions indicate skipping of both the inverse quantization and the inverse transform; inversely quantizing the obtained residual data and skipping the inverse transform for the inversely quantized residual data to thereby reconstruct a residual block using the inverse- quantized residual data, when the one or more identified conditions indicate skipping of the inverse transform; inversely quantizing the obtained residual data and inversely transforming the inversely quantized residual data to thereby reconstruct a residual block using the inversely transformed residual data, when the one or more identified conditions indicate non-skipping of the inverse quantization or the inverse transform; and adding each pixel value of the predicted block to a corresponding pixel value of the reconstructed residual block.