In this study, we aim at clarification of the factor that promotes an user's acceptance of suggestion from an interactive agent in driving environment. Our aim is to figure out how human beings accept the encouragement from interaction objects, and also which kinds of dialogues or action controls are necessary for the design of car navigation system which makes suggestion and requests to drivers. Firstly, we had an experiment for collecting dialogue between humans in driving simulation environment, then we analyzed the drivers' acceptance and evaluation for the navigators. As the results, we found that the presence and reliability of the navigator highly relate to the acceptance of suggestion from the navigator. When navigators were next to drivers, the rate of drivers' suggestion acceptance rose. However, the stress of drivers increased. In addition, based on the linguistic and acoustic analysis of the navigators' utterances, we found out some points of designing system utterance of suggestion to promote user's acceptance. We found that expressing the grounds of suggestions, showing the exact numbers, and the wide pitch ranges, all highly relate to the acceptance of suggestions.