A video signal of an interlaced scanning system having 525 scanning lines and a vertical scanning frequency of 60 Hz is converted to a video signal having 1050 scanning lines and a vertical scanning frequency of 120 Hz, for displaying an image by bidirectional scanning. A vertical synchronizing signal is subjected to offset processing by a 1/4 horizontal scanning period when an odd field is started, thereby keeping interlaced relation between odd and even fields. The vertical synchronizing signal is subjected to offset processing by a 1/2 horizontal scanning period every frame, so that the scanning direction for each scanning line is reversed every frame.