The purpose of this study is to analyse the relationship between a driver's cognition of changes in traffic circumstances and his driving maneuver and to understand the driving maneuver. Therefore，the basic experiment using tow cars and the experiment which a subject drives a car in real traffic flow were conducted. And in both experiments subjects were asked about cognition of a visual information on traffic flow. And moreover the methods of those experiments were discussed. The obtained knowledge are summarized as follow. Firstly the vehicular gap under car following is different between before and after acceleration of a car running ahead. Secondly the vehicular gap is influenced with a driver's cognition of visual information.