A double-rate signal achieved by subjecting a video signal to double-rate conversion is supplied to a scan line number converter. In the converter, the portion of the effective scan lines of the double-rate signal is written into a frame memory on the basis of a signal achieved by multiplying horizontal and vertical synchronous signals based on the double-rate signal. In the effective scan line section of HDTV signal, the video signal written in the frame memory is read out on the basis of horizontal and vertical reference signals based on the HDTV signal. Out of the effective scan line section of the HDTV signal, a pedestal level signal written in a memory is read out on the basis of the horizontal and vertical reference signals based on the HDTV signal, thereby achieving HDTV signal whose vertical scan line number is equal to 1125 lines. Accordingly, the HDTV signal suffers no degradation because the video signal in the section of 960 effective scan lines of the double-rate signal is not interpolated.