A control apparatus for a steering system, the steering system being configured to turn turning wheels of a vehicle and including an electric motor, the control apparatus being configured to control the steering system, and comprising: control circuitry configured to perform an angle control process for controlling, to an angle command value, a convertible angle that is convertible into a rotational angle of the electric motor, a predetermined component calculation process for calculating a predetermined component containing at least one of two components that are a viscosity component and a friction component of the steering system, while using a value of a variable regarding a control quantity of the electric motor as an input, and a relationship change process for changing a relationship of an output from a control unit (M60, M80, M82; M42, M44) for the steering system to an input to the control unit (M60, M80, M82; M42, M44), based on the predetermined component calculated through the predetermined component calculation process, the control apparatus characterised in that the predetermined component calculation process includes a viscosity component calculation process for calculating the viscosity component as the predetermined component; and the relationship change process includes a process for changing the relationship of the output from the control unit (M60, M80, M82; M42, M44) for the steering system to the input to the control unit (M60, M80, M82; M42, M44), based on the viscosity component calculated through the viscosity component calculation process, the viscosity component calculation process includes a disturbance torque calculation process for calculating, as a disturbance torque, a torque component influencing the convertible angle other than a torque of the electric motor, while using a value of a variable regarding the torque of the electric motor, a detection value of the convertible angle, and the angle command value as values of variables regarding the control quantity, and the viscosity component calculation process includes a filtering process for allowing selective transmission of a specific frequency component of the disturbance torque, while using the disturbance torque as an input, and the viscosity component calculation process uses an output of the filtering process as the viscosity component. control circuitry configured to perform an angle control process for controlling, to an angle command value, a convertible angle that is convertible into a rotational angle of the electric motor, a predetermined component calculation process for calculating a predetermined component containing at least one of two components that are a viscosity component and a friction component of the steering system, while using a value of a variable regarding a control quantity of the electric motor as an input, and a relationship change process for changing a relationship of an output from a control unit (M60, M80, M82; M42, M44) for the steering system to an input to the control unit (M60, M80, M82; M42, M44), based on the predetermined component calculated through the predetermined component calculation process, the control apparatus characterised in that the predetermined component calculation process includes a viscosity component calculation process for calculating the viscosity component as the predetermined component; and the relationship change process includes a process for changing the relationship of the output from the control unit (M60, M80, M82; M42, M44) for the steering system to the input to the control unit (M60, M80, M82; M42, M44), based on the viscosity component calculated through the viscosity component calculation process, the viscosity component calculation process includes a disturbance torque calculation process for calculating, as a disturbance torque, a torque component influencing the convertible angle other than a torque of the electric motor, while using a value of a variable regarding the torque of the electric motor, a detection value of the convertible angle, and the angle command value as values of variables regarding the control quantity, and the viscosity component calculation process includes a filtering process for allowing selective transmission of a specific frequency component of the disturbance torque, while using the disturbance torque as an input, and the viscosity component calculation process uses an output of the filtering process as the viscosity component.