An apparatus is disclosed to convert a non-interlaced computer graphics signal which consists of contiguous scan lines into an interlaced video signal which alternately consists of even fields and odd fields. The converting apparatus includes a receiving means, a low-pass filter and a line buffer. The receiving means sequentially receives a scan line. The line buffer stores the brightness signals of the first and the second scan lines before the received scan line. And the low-pass filter receives the sequential contiguous scan lines from the line buffer and the displaying scan line, weights the brightness signals of these scan lines and outputs the scan lines with the weighted brightness signal and a halved refresh rate to serve as the even field and the odd field of the interlace video signal.