In order to reduce the size of a data stream of image data transmitted via a network and to efficiently manage such a data stream, the image data is coded in a highly efficient fashion using a hierarchical coding algorithm, such as a wavelet transformation. When a data stream is being inputted, the data stream is analyzed and input of the data stream is interrupted in accordance with an analysis result. An interrupted stream is generated from the data stream that has already been inputted. The resultant interrupted stream is stored.