In recent years, researches and developments on ITS are carried out to solve many problems in road traffic. IVC (Inter-Vehicular Communications) is the key technology to realize the next generation ITS. Vehicles install telecommunication equipment, and exchange safety information directly with each other. The authors have developed Vehicular Collision Avoidance Suppor t System (VCASS) based on IVC in order to prevent a vehicular accident beforehand. The system grasps the relative locations of vehicles by exchanging the GPS information in each vehicle. The system warns drivers if there is a danger of collision. VCASS can drastically reduce the potential accidents of vehicular collisions. However, VCASS makes false warnings and unnecessary warnings due to the position estimation error of vehicles. In this paper we propose an extended collision judgment algorithm in order to solve these problems of VCASS. We evaluate the performances of conventional and proposed methods with real vehicles.