A cooling system comprising: a cooling device for cooling a power control unit of a vehicle, wherein the cooling device includes a rotating component for delivering cooling medium and a drive source for controlling the rotating component; and an electronic control device for controlling the cooling device, wherein the electronic control device is adapted to store drive source diagnosis reference information that indicates an actual rotation speed acceptable range when an actual rotation speed of the drive source is apart from an instructed rotation speed of the drive source, wherein the actual rotation speed acceptable range is an acceptable range of the actual rotation speed, the acceptable range changes according to the instructed rotation speed and a voltage value applied to the drive source, the drive source diagnosis reference information comprises a map of an instructed rotation speed and an actual rotation speed acceptable range corresponding to the instructed rotation speed for the voltage value, and the actual rotation speed acceptable range is wider along with the increase of the instructed rotation speed, obtain the acceptable range of the actual rotation speed from the voltage value applied to the drive source and the instructed rotation speed based on the drive source diagnosis reference information, when the actual rotation speed of the drive source is included in the obtained acceptable range, determine that the cooling device is in a normal state, and when the actual rotation speed is not included in the acceptable range, determine that the cooling device is in an abnormal state. a cooling device for cooling a power control unit of a vehicle, wherein the cooling device includes a rotating component for delivering cooling medium and a drive source for controlling the rotating component; and an electronic control device for controlling the cooling device, wherein the electronic control device is adapted to store drive source diagnosis reference information that indicates an actual rotation speed acceptable range when an actual rotation speed of the drive source is apart from an instructed rotation speed of the drive source, wherein the actual rotation speed acceptable range is an acceptable range of the actual rotation speed, the acceptable range changes according to the instructed rotation speed and a voltage value applied to the drive source, the drive source diagnosis reference information comprises a map of an instructed rotation speed and an actual rotation speed acceptable range corresponding to the instructed rotation speed for the voltage value, and the actual rotation speed acceptable range is wider along with the increase of the instructed rotation speed, obtain the acceptable range of the actual rotation speed from the voltage value applied to the drive source and the instructed rotation speed based on the drive source diagnosis reference information, when the actual rotation speed of the drive source is included in the obtained acceptable range, determine that the cooling device is in a normal state, and when the actual rotation speed is not included in the acceptable range, determine that the cooling device is in an abnormal state. store drive source diagnosis reference information that indicates an actual rotation speed acceptable range when an actual rotation speed of the drive source is apart from an instructed rotation speed of the drive source, wherein the actual rotation speed acceptable range is an acceptable range of the actual rotation speed, the acceptable range changes according to the instructed rotation speed and a voltage value applied to the drive source, the drive source diagnosis reference information comprises a map of an instructed rotation speed and an actual rotation speed acceptable range corresponding to the instructed rotation speed for the voltage value, and the actual rotation speed acceptable range is wider along with the increase of the instructed rotation speed, obtain the acceptable range of the actual rotation speed from the voltage value applied to the drive source and the instructed rotation speed based on the drive source diagnosis reference information, when the actual rotation speed of the drive source is included in the obtained acceptable range, determine that the cooling device is in a normal state, and when the actual rotation speed is not included in the acceptable range, determine that the cooling device is in an abnormal state.