An input digital information signal is prediction-encoded and thereby difference signals are generated. The difference signals are block segmented. A maximum value and a minimum value of each block are detected. Whether or not the range of levels of the maximum value MAX and the minimum value MIN crosses 0, a mode determining circuit 5 determines a quantizing mode and generates a mode signal MODE that represents the determined quantizing mode. When the range of the levels crosses 0, a first quantizing mode is selected. Otherwise, a second quantizing mode is selected. In the first quantizing mode, the quantizing circuit 6 performs the normal quantizing process. In the second quantizing mode, the quantizing circuit 6 performs a code conversion in such a manner that the number of bits of quantized output data is smaller than that of the normal quantizing process.