An A/D conversion apparatus of a digital video system includes an A/D converter for converting an input analog video signal to a digital signal on the basis of a reference voltage value input from the outside and outputting the digital signal; and a reference voltage supplying portion for supplying the A/D converter with a predetermined number of the reference voltage value sequentially and repeatedly during a predetermined time period. Accordingly, since the quantization error can be minimized during the A/D conversion in the digital video system without increasing the number of the quantization bits, the image quality can be improved.