The distance control assist system with an active gas pedal and a deceleration control was developed for the purpose of assisting a driver to maintain following distance to a lead vehicle. A field test was conducted to evaluate the effectiveness of this system. The result showed that, frequency and magnitude of deceleration by the driver were decreased, and, from the characteristics of TTC(Time to Collision)distribution, frequency of closing situations were decreased, that might contributed to the reductin of brake action by the driver. The AWWL(Adapted Weighted Workload)score on the NASA-TLX(Task Load Index)and the salivary amylaze activity score were also analyzed and the results showed the decrease of driver's workload by using the system.