We can see frequently lane changing phenomena on multiple lane roads，these phenomena are one of the dominant factors disturbing traffic flow. Therefore，the purpose of this study is to discuss on a driver's cognition of visual information with fuzziness and the process of decision making of lane changing. For this reason，questionnaire surveys and driving experiments were conducted. The obtained knowledge are summarized as follow. Firstly drivers who will change a lane are divided into three types such as considering informations of vehicular gap，running speed and both. And the driver's rule of lane changing was built.