To perform digital data compression by converting digital data through a discrete wavelet conversion to convert digital data, generating corresponding conversion data, and quantizing the conversion data by using a variable quantization method determined by a corresponding quad tree structure.

An input picture 102 indicated in a high level block diagram is preferably inputted to a conversion block 110 for performing linear conversion and generates a corresponding conversion coefficient 112. Here, a discrete/ wavelet transformation(DWT), a discrete cosine transformation (DCT) or the like is adapted. The conversion coefficient 112, or to be more specific, a bit sequence for representing its value, is encoded by a bit restructure block 120 using an efficient method in order to obtain an encoding representation 122.