The present invention is directed to an image decoding apparatus adapted for decoding information obtained by implementing inverse quantization and inverse orthogonal transform to image compressed information in which an input image signal is blocked to implement orthogonal transform thereto on the block basis so that quantization is performed with respect thereto, which comprises a reversible decoder (12) for decoding quantized and encoded transform coefficients, an inverse quantizer (13) indicating, as a flag, in inverse-quantizing transform coefficients which have been decoded by the reversible decoder (12), existence of each transform coefficient every processing block of inverse quantization, and an inverse transform element (14) for changing inverse transform processing to be implemented to inverse quantization transform coefficients within processing block by using the flag which has been indicated by the inverse-quantizer (13).