This study investigated which factors affect the usability evaluation of an ACC (Adaptive cruise control). In the experiment, the participants drove a Toyota Prius car with an ACC on real highway. 215 different kinds of driving data were sampled at 60 Hz during driving. At each of the six designated stop points set on driving course, the participants stopped the car and evaluated the usability of the ACC by answering Usability Questionnaire for Automation System. The participants’ driving styles were measured with Driving Style Questionnaire. As results of multiple regression analyses,we found how the participants’ driving styles, the driving control of the ACC, and the participants’ intervention in the driving control of the ACC influenced the usability evaluation. The results were discussed in terms of the human-automation interaction and the design principles of an ACC.