This paper investigates the driver's cognitive process when using In-Vehicle Navigation System (IVNS) by means of verbal protocol analysis which has been used in cognitive psychology and recently adopted as a technique for studying user behavior in Human-Computer Interaction. This investigation consists of field experiments and laboratory experiment. A concept of cognitive maps was used as frameworks of the analysis. In the first stage, assuming that a person who is familiar with an area is the best navigator, communication between a driver and a navigator was observed. Three prototypes of IVNS were designed by considering the observations mentioned above, and comparative studies were conducted. The results indicate important factors to design the user-friendly interface of IVNS such as: Drivers concentrate their attentions on which corners they should turn next, that is, they establish sub-goals to achieve the whole goal. Navigational information should be presented for each sub-goal. Navigational information should consist of landmarks, paths and nodes which are part of so called five components of the cognitive map. Especially, landmarks should be enhanced in the CRT to let driver take his own strategy easily.