An image coding method includes the steps of producing a complexity normalizing signal for respective macro blocks based on a complexity in a space domain with regard to an input image, producing a zero coefficient sum by performing a discrete cosine transform operation on the input image and counting the discrete cosine transform coefficients from -1 to +1, producing a zero coefficient normalizing value for respective macro blocks in accordance with the zero coefficient sum; producing a transient weight signal for respective macro blocks in accordance with the zero coefficient sum and a predetermined compression ratio, and summing the complexity normalizing signal, the zero coefficient normalizing value and the transient weight signal so as to produce the scale factor. The quantization step size of the discrete cosine transform coefficients is controlled in a macro block in accordance with the scale factor. Thus, image frames having various complexities can be encoded at a constant bit rate with no deterioration of the image.