On-board voice interaction systems such as voice activation system or text-to-speech (TTS) system enable drivers to operate devices or to obtain the desired information without relying on their visual processes. Although these systems are thought to reduce the driver’s workload, few papers published papers have quantified any reduction. This paper describes the tests that were carried out to determine its potential influence on drivers' mental distraction. Tests were conducted with a driving simulator and with an actual vehicle. Subjects were given various on-board device tasks to operate while driving. Their eye motion, response time to the illumination of the LED installed in a vehicle and vehicle behavior data were recorded when the tasks were performed. The results obtained for all of the indices show that the mental distraction level when listening to a TTS reading of information is comparable to that of listening to the car radio and the workload of the voice activation system is significantly lower than that of a traditional manual operation system.