To provide a preventive device for preventing such an occurrence that the load on an outrigger varies in association with erroneous operation of the jib of a truck crane when objects are loaded, unloaded, or carried by the crane, to intrude into the hazardous range for tumbling, and that the truck finally loses balance and tumbles to lead to damage of the crane and/or truck.

A pressure sensor 14 is inserted between the outrigger and grounding surface of a truck crane so as to measure the varying amount of the load in association with manipulation of the jib of the crane, and alarm with display and voice is emitted in the initial stage of the hazardous range in which the truck loses balance and tumbles. An appropriate control signal is given to the crane operating device to generate automatic stop, and thereby the truck is precluded from risk of tumbling.