1. 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, the image decoding apparatus comprising: decoding means for decoding quantized and encoded transform coefficients; inverse-quantizing means for indicating, as a flag, in inverse-quantizing the transform coefficients which have been decoded by the decoding means, existence of each transform coefficient every processing block of inverse-quantization; and inverse transform means for changing inverse transform processing to be implemented to inverse quantization transform coefficients within the processing block by using the flag which has been indicated by the inverse quantization means.