Abstract: The overturning-preventing device (1) for crane trucks (3) and similar machines is associated with the self-propelled truck (6) of the crane truck (3) in correspondence of the rear axle (8) of the same car, and is substantially composed by a pair of plate means (20, 21) hinged to each other (22) in correspondence of one of their edges. The necessary force in order to generate the rotation of the plate means (20, 21) relatively to each other is adjustable by means of suitable adjustment means (28, 28A) which influence the minimum load transmitted by the rear axle (8) to the ground. In case the overturning torque is large, the rear portion of the self-propelled truck (6) tends to lift, while the rear axle (8), thanks to its own weight, tends to remain stably resting on the ground, causing the plate means (20, 21) of the overturning-preventing device (1) to divaricate. Such a divarication is used as a signal for enabling a circuit commanding the load-lifting operations by the crane truck ( 3) to be discontinued.