Abstract: In a scanning line number converting device, a video signal is converted into a digital signal, its scanning lines being thinned out by a thinning-out circuit (5) in the vertical direction to reduce the vertical direction sampling frequency and interpolated so as to form a non-interlace signal. The non-interlace signal is written in a frame memory (7), under control of a writing control circuit (8). A field determination circuit (15) determines a field of a video signal used as reference. According to this determination output, the non-interlace signal is read out of the frame memory while undergoing a conversion into an interlace signal such that its interlace sequence corresponds to that of the reference video signal, resulting in a conversion of the scanning line number. A down-converter for converting a high-resolution TV signal into an NTS signal is configured using this scanning line number converting device.