An apparatus interpolates color-difference line-sequential signals appearing alternately in a video signal together with a luminance signal. The video signal is received by an input circuit and then held temporarily and on a horizontal scanning line basis by a hold circuit. Each of the color-difference signals is interpolated by an interpolating circuit by producing an arithmetic mean of the color-difference data of the video signal being held by the hold circuit and color-difference data of the video signal being received by the input circuit, on the basis of the pixels corresponding to each other on horizontal scanning lines.