PURPOSE: To reduce a wheel set by the self-excited oscillation of a driving system by providing a trunk frame or gear box with a sensor and by compensating a driving force, when a signal within a certain range including a frequency continues in excess of a preset output.

CONSTITUTION: A sensor 11 is provided at a trunk frame 10 or gear box 8 so that an oscillation generated by slip is detected quickly. After signals detected by the sensor 11 were amplified by an amplifier 12, unnecessary signals are removed by a bandpass filter 13 and the remaining signals are sent to a slip detection circuit 3. In the slip detection circuit 3, a logical operation is conducted as to whether an input signal is a self-excited oscillation or not. According to the results of the logical operation, a driving force is compensated so that the oscillation converges smoothly.