PURPOSE: To contrive improvement of safety at the time of hoisting work with crane or the like by generating an alarm in the case of floating an outrigger detached from the surface of the ground at the time of hoisting work by the crane.

CONSTITUTION: An internal pressure of a hydraulic jack 12 for an outrigger 10 is transmitted to a pressure switch 22 through a pressure oil hose 20 with an intermediate block 18 serving as detecting end. The positive internal pressure is surely generated in the hydraulic jack 12 by reaction force of the ground surface in a normal condition, and by moving a crane tilted, the jack internal pressure partly in the outrigger 10 is gradually decreased to become 0 when floating is started. For instance in the pressure switch 22, its working point is set to 30kg/mm2 to reach a preset value just before the outrigger 10 is started to float. As a result, the pressure switch 22 is actuated to predict a hazard by generating an alarm by an alarm device 24. In this way, safety is improved at the time of hoisting work in the crane or the like.