A rounding accuracy of DCT coefficients that are quantized is dynamically varied with a simple structure. As a result, an encoding noise is effectively suppressed and the picture quality is prevented from being deteriorated. DCT coefficients of which a picture signal is transformed by DCT are input to a quantizing circuit 30. The quantizing circuit 30 quantizes the DCT coefficients with a quantizer matrix and supplies the quantized DCT coefficients to a multiplying device 33 through an absolute value circuit 31. A quantizer scale value based on the generated code amount of the entire screen is converted into an invert value thereof and then supplied to a multiplying device 33. In addition, an M value designating circuit 38 designates a value M as a parameter with which a rounding accuracy of DCT coefficients that are quantized is controlled according to a quantizer scale value. An adding device 34 adds the value M to an output of the multiplying device 33. A five-bit shifting circuit 35 rounds off a decimal part. At that point, as the quantizer scale value becomes large, the rounding accuracy of DCT coefficients that are quantized is designated so that the round off direction becomes large. Thus, the DCT coefficients are quantized so that the amplitude is not increased. As a result, the encoding noise can be suppressed.