A control device for a steer-by-wire steering device having a redundancy function for controlling the steer-by-wire steering device; the steer-by-wire steering device comprising: a turning shaft having its left and right ends provided with respective tie rods, a steering wheel, a reactive force actuator to apply a steering reactive force to the steering wheel, a first steering angle sensor and a second steering angle sensor to detect respective steering angles of the steering wheel, a main motor and a sub motor, a turning power transmission mechanism to transmit a rotation of the main motor to the turning shaft, a toe angle adjusting power transmission mechanism to adjust the toe angles by means of a rotation of the sub motor; and a switch mechanism capable of switching to an abnormal case switched mode by detaching the main motor from the turning power transmission mechanism and transmitting the rotation of the sub motor to the turning power transmission mechanism to enable a turning; characterized in that the control unit comprising a first control device and a second control device, which communicate with each other; the first control device including a basic control function unit to control the reactive force actuator and the sub motor upon receipt of a detection signal of the first steering angle sensor; the second control device including a basic control function unit to control the main motor upon receipt of a detection signal of the second steering angle sensor; each of the first and second control devices (101, 201) including a mutual trouble diagnosis unit (103, 203) to diagnose a mutual troubles; the second control device including a main motor failure diagnosis unit to diagnose a failure of the main motor and to transmit a result of diagnosis to the first control device; the first control device including an abnormal case switch command unit to command the switch mechanism to switch to the abnormal case switched mode when the first control device receives from the second control device a result of diagnosis that the main motor fails or when the mutual trouble diagnosis unit diagnoses that the second control device is in a trouble. a turning shaft having its left and right ends provided with respective tie rods, a steering wheel, a reactive force actuator to apply a steering reactive force to the steering wheel, a first steering angle sensor and a second steering angle sensor to detect respective steering angles of the steering wheel, a main motor and a sub motor, a turning power transmission mechanism to transmit a rotation of the main motor to the turning shaft, a toe angle adjusting power transmission mechanism to adjust the toe angles by means of a rotation of the sub motor; and a switch mechanism capable of switching to an abnormal case switched mode by detaching the main motor from the turning power transmission mechanism and transmitting the rotation of the sub motor to the turning power transmission mechanism to enable a turning; the first control device including a basic control function unit to control the reactive force actuator and the sub motor upon receipt of a detection signal of the first steering angle sensor; the second control device including a basic control function unit to control the main motor upon receipt of a detection signal of the second steering angle sensor; each of the first and second control devices (101, 201) including a mutual trouble diagnosis unit (103, 203) to diagnose a mutual troubles; the second control device including a main motor failure diagnosis unit to diagnose a failure of the main motor and to transmit a result of diagnosis to the first control device; the first control device including an abnormal case switch command unit to command the switch mechanism to switch to the abnormal case switched mode when the first control device receives from the second control device a result of diagnosis that the main motor fails or when the mutual trouble diagnosis unit diagnoses that the second control device is in a trouble.