To reduce the occurrence of a pseudo contour and to obtain an excel lent image quality by preventing continuous occurrence of a single quantization representative value in any input signal value.

In the case of using plural quantization values for an input signal and generating and outputting a value finner than a quantization interval based on an incidence ratio, the input signal is classified into plural sets, e.g. four ranges depending on the value of the input signal, a quantization representative value D not a member of the classified set I (D belongs to 1), for example, a quantization representative value 170 is used for a signal within a range of 64-128 and a quantization representative value 85 is used for a signal within a range of 128-192 for quantization.