Turn-taking in a smooth conversation is supported by the anticipation of the floor handover timing among participants. However, it becomes difficult to maintain natural turn-taking in video conferencing with transmission delays because the utterances and movements of each participant are presented to the others with a time lag, which often leads to a collision of utterances. In order to facilitate smooth communication over a video-conferencing system, we propose a novel method, "Visual Filler," that fills temporal gaps in turn-taking caused by the existence of delays. Visual Filler overlays an artificial visual stimulus that has a function similar to that of filler sounds on a screen with participant images. We have evaluated the effectiveness of a Visual Filler for reducing the unnaturalness of turn-taking on a simulated dyadic dialog situation with a delay.