PURPOSE: To lock the operation of incliation indication when the acceleration of a vehicle is higher than a prescribed value by generating a signal which locks a driving source when the acceleration signal converted from the speed signal of a speed sensor exceeds a set value.
CONSTITUTION: The pulse signal corresponding to the speed of a vehicle from a speed sensor 20 is inputted through a smooting circuit 24 of a control circuit 23 to a differentiating circuit 25, from which the signal corresponding to the acceleration of the vehicle is outputted. Said signal is compared with a set value in a switching circuit 27 through an amplifying circuit 26, and when the same is smaller than the set value, the switching circuit does not emit any output signal. Therefore, the inclinometer indicates an inclination. On the other hand, when the signal from the circuit 26 is larger than the set value, the circuit 27 feeds a signal to a locking mechanism 28, which in turn energizes a solenoid part 29 so that the driving source is locked by attacting a lock actuating member 30, whereby the operation of the inclination indication is locked.