A steer-by-wire steering system, comprising: an operation member to be operated by a driver; a two-system reaction force applying device including two reaction force controllers corresponding to two systems, the reaction force applying device being configured to, in each of the two systems, obtain operation information that is information relating to an operation degree of the operation member and apply an operation reaction force to the operation member based on the operation information; a two-system steering device including two steering controllers corresponding to two systems, the steering device being configured to, in each of the two systems, steer a wheel based on the operation information; an operation information obtaining device configured to obtain the operation information independently of the reaction force applying device ; an auxiliary steering device capable of changing a direction of a vehicle on which the steer-by-wire steering system is installed, independently of the steering device ; two dedicated communication lines one of which information-transmittably and information-receivably connects one of the two reaction force controllers and one of the two steering controllers to each other, and the other of which information-transmittably and information-receivably connects the other of the two reaction force controllers and the other of the two steering controllers to each other; and a first communication bus to which the operation information obtaining device is at least information-transmittably connected and to which the two steering controllers and the auxiliary steering device are at least information-receivably connected. an operation member to be operated by a driver; a two-system reaction force applying device including two reaction force controllers corresponding to two systems, the reaction force applying device being configured to, in each of the two systems, obtain operation information that is information relating to an operation degree of the operation member and apply an operation reaction force to the operation member based on the operation information; a two-system steering device including two steering controllers corresponding to two systems, the steering device being configured to, in each of the two systems, steer a wheel based on the operation information; an operation information obtaining device configured to obtain the operation information independently of the reaction force applying device ; an auxiliary steering device capable of changing a direction of a vehicle on which the steer-by-wire steering system is installed, independently of the steering device ; two dedicated communication lines one of which information-transmittably and information-receivably connects one of the two reaction force controllers and one of the two steering controllers to each other, and the other of which information-transmittably and information-receivably connects the other of the two reaction force controllers and the other of the two steering controllers to each other; and a first communication bus to which the operation information obtaining device is at least information-transmittably connected and to which the two steering controllers and the auxiliary steering device are at least information-receivably connected. <EOS>