Abstract
This study examines the influence of driver's attention when using driving assistance system on Forward Collision Warning System (FCWS) by driving simulator experiments. Driver's brain activity is measured by near-infrared spectroscopy (NIRS). We conducted an experiment that subject followed a leading vehicle which suddenly decelerated. The brain activity of the driver without FCWS was higher than that of the driver with FCWS. Significant difference between following a leading vehicle with FCWS and without FCWS can be observed. This result is caused by the difference of the driver's attention to a leading vehicle.