Vehicular collision avoidance radar systems using millimeter-wave have been developed in recent years. Because of their simple structures, a microstrip antenna or a lens antenna is often used as the radar antenna. However, the antenna efficiencies of these antennas are low, in general. Therefore, the types of antenna that have small antenna aperture are difficult to obtain high antenna gains. In addition, it is difficult to radiate many beams at narrow beam intervals with a lens antenna.We developed an offset paraboloidal reflector antenna (the OP antenna) for the vehicular collision avoidance radar. This antenna consists of one pyramidal horn antenna and one offset paraboloidal reflector. They pyramidal horn and the radio frequency unit (the RF unit) of the radar are fixed in the radar head. To scan the beam, only the reflector is rotated. Using a reflector-rotating mechanism, the OP antenna can radiate many beams toward different directions. Therefore, it is easy for the OP antenna to increase the number of beams and to extend the target detection angle range. We have improved the OP antenna in order to extend the target detection angle range and to miniaturize the radar head. The OP antenna was designed to radiate four beams so that the target detection angle range might become 12 degrees.We have produced the OP antenna and measured its characteristics. For each four beams, the antenna gain of 28 dBi and the antenna efficiency of 63% are obtained. Both measured results of the radiation patterns and the antenna gains are almost corresponding to the calculated results. Consequently, it was confirmed that the OP antenna is useful for vehicular collision avoidance radar systems. The radar system using the OP antenna is now in the experimental stage on vehicle to verify and improve the radar performances.