Nowadays, more and more technologies are implemented in vehicles to improve driver assistance in order to enlarge comfort and avoid accidents. Automotive radar systems have been gradually installed into vehicles with the growth of wireless technologies. Among the utilizations of automotive radar systems, survey around automobiles for the prevention of traffic accidents is one of the prospective applications. Especially, in the millimeter-wave automotive radar systems, the microstrip antennas are superior candidates when radar sensors are widely used in vehicle due to its advantages of low cost and low profile. Generally microstrip antennas are placed on the surface of the radar sensor and are connected to millimeter-wave circuits inside of the sensor via waveguides. Therefore, transitions from waveguide to microstrip line are required and their key devices. Furthermore, transitions were one of the important devices which connect different types of transmission lines used to transport millimeter-wave signals for not only millimeter-wave radar systems but also many other millimeter-wave systems.