The maximum frequency and the minimum frequency are extracted from frequencies corresponding to respective axle speeds. The minimum frequency is subtracted from the maximum frequency to achieve a first frequency deviation, and also the first frequency deviation is input as a primary delay system to achieve a second frequency deviation. Furthermore, the second frequency deviation is subtracted from the first frequency deviation to achieve an idle running detection setting deviation, and also an idle running detection set value for judging the idle running of the wheels at the frequency level is achieved. The idle running frequency deviation and the idle running detection set value are compared with each other to output an idle running detection signal, and also a torque correction amount of an AC electric motor is calculated on the basis of the idle running detection signal.