A cooling system of an internal combustion engine includes a control valve that regulates the flow of a coolant in a circulation circuit, and an electronic control unit. The electronic control unit is configured to have the following functions: controlling the driving of a motor of the control valve; calculating a motor torque based on an effective voltage applied to the motor; calculating a valve body torque that is part of the motor torque, based on an angular acceleration rate of the motor; and calculating a driving stress based on a difference between the motor torque and the valve body torque.