Data compression apparatus

In a data compression apparatus, input data are subjected to a predetermined filtering process. A predetermined calculation is executed on data which result from the filtering process. The predetermined calculation includes at least an addition. The result of the predetermined calculation is converted into a corresponding predicted code amount. A target code amount is set. A difference between the predicted code amount and the target code amount is derived, and a quantization step size is generated in correspondence with the difference. The input data are subjected to a predetermined orthogonal transform. Data which result from the orthogonal transform are quantized with the generated quantization step size.