A steering control system that controls steering of a vehicle, the steering control system comprising: an actuator configured to apply force to a member associated with the steering, the actuator having a duplex configuration including a first actuator of a main system and a second actuator of a subsidiary system; and a control device configured to control the actuator, the control device having a duplex configuration including a first control device that belongs to the main system, and a second control device that belongs to the subsidiary system, the first control device and the second control device being configured to compute the same controlled variable as a first controlled variable and a second controlled variable, respectively, wherein the control device has operating modes including a normal mode in which at least one of the first actuator and the second actuator is controlled according to the first controlled variable, without using the second controlled variable, and an independent mode in which the first control device controls the first actuator according to the first controlled variable, and the second control device controls the second actuator according to the second controlled variable, the first control device and the second control device are configured to communicate with each other in the normal mode, in the normal mode, the first control device is configured to send the first controlled variable to the second control device, and the second control device is configured to send the second controlled variable to the first control device, and the control device is configured to switch the operating mode from the normal mode to the independent mode, in at least one of a case where a discrepancy arises between the first controlled variable and the second controlled variable, and a case where a communication abnormality occurs in communications between the first control device and the second control device; and wherein: each of the first control device and the second control device has a self-diagnosis function of detecting an abnormality in itself; the operating modes of the control device further include a backup mode in which, when occurrence of the abnormality in one of the first control device and the second control device is confirmed through the self-diagnosis function, the other of the first control device and the second control device controls the actuator; and a start time of the independent mode is earlier than that of the backup mode. an actuator configured to apply force to a member associated with the steering, the actuator having a duplex configuration including a first actuator of a main system and a second actuator of a subsidiary system; and a control device configured to control the actuator, the control device having a duplex configuration including a first control device that belongs to the main system, and a second control device that belongs to the subsidiary system, the first control device and the second control device being configured to compute the same controlled variable as a first controlled variable and a second controlled variable, respectively, wherein the control device has operating modes including a normal mode in which at least one of the first actuator and the second actuator is controlled according to the first controlled variable, without using the second controlled variable, and an independent mode in which the first control device controls the first actuator according to the first controlled variable, and the second control device controls the second actuator according to the second controlled variable, the first control device and the second control device are configured to communicate with each other in the normal mode, in the normal mode, the first control device is configured to send the first controlled variable to the second control device, and the second control device is configured to send the second controlled variable to the first control device, and the control device is configured to switch the operating mode from the normal mode to the independent mode, in at least one of a case where a discrepancy arises between the first controlled variable and the second controlled variable, and a case where a communication abnormality occurs in communications between the first control device and the second control device; and the operating modes of the control device further include a backup mode in which, when occurrence of the abnormality in one of the first control device and the second control device is confirmed through the self-diagnosis function, the other of the first control device and the second control device controls the actuator; and a start time of the independent mode is earlier than that of the backup mode.