In a motor cooling system 1, a determination unit 33 is configured to perform periodic sampling of a driving current Ip or a rotation speed Np of an oil pump 23, and detect a decrease in cooling oil of a motor 2 when it is determined that a standard deviation of the driving current Ip or the rotation speed Np with respect to a plurality of sampling points is larger than a preset threshold value. The threshold value is set on the bases of a measurement value of the driving current Ip or the rotation speed Np of the oil pump 23 which is obtained by an operation test of the motor 2 in a state in which the cooling oil has decreased.