A bad editing detection device is provided to pre-detect a bad editing of image signal, so as to avoid zigzag effect in the television image. When the input image is from a film, instead of from a video source, the line doubler performs a de-interlace for the film signal to increase the vertical resolution of the television signal. The detection device detects a bad editing before the image signal is played and adjusts the de-interlace of the line doubler to avoid zigzag effect.