Information providing service has been started in Japan by using ITS Spot (newly developed 5.8GHz DSRC V2I communication system) since 2011. National Institute for Land and Infrastructure Management (NILIM) has developed the Safe Driving Support System at Curve Section as part of its service; voice guidance is provided through ITS Spot. This system has been installed at Kumano-cho Curve (a part of Kumano-cho Junction) and voice guidance has been provided by means of ITS Spot service. This paper will report the results of the field operational tests (FOTs) conducted to verify the effectiveness of the system. The effectiveness of Safe Driving Support System was identified by comparing driving behaviors with and without voice guidance as well as questionnaire survey against drivers of the FOTs.