To reliably detect the operation state of a drive train by determining the operation state based on the ratio between the engine revolving speed and vehicle speed and the aging change of this ratio.

An electronic control unit 10 reads the engine revolving speed N and vehicle speed V and forms a ratio as a quotient of them, and it forms an aging change based on the measured values of the ratios at the different time. When the ratio is on the outside of the tolerance range for the gear position or when the value of the aging change is extremely larger than the prescribed threshold value, the transition from a closed clutch to an opened clutch is detected. When the value of the aging change is smaller than the threshold value and the ratio exists within the prescribed tolerance range, the transition from the opened clutch to the closed clutch is detected. If an incorrect decision is made in this detecting method, it is automatically corrected, thus no effect is given to the control of a drive unit by the incorrect decision.