A decoding apparatus for decoding an encoded image signal and its control method. The decoding apparatus decodes the encoded image signal and inverse-quantizes decoded information. The apparatus inverse-orthogonal transform an inverse-quantized information to obtain a decoded image signal. A quantization error estimator estimates a maximum square error of the inverse-quantized information and a second inverse-orthogonal transformer inverse-orthogonal transforms the quantization error from the quantization error estimator. An image-quality enhancing signal generator generates an image-quality enhancing signal based on the quantization maximum error and the decoded image signal, and an image synthesizer generates a high-quality image signal by synthesizing the image-quality enhancing signal with the decoded image signal.