31a and steel balls 30 the striker 31a has sealing members 34a and 34b hereinafter the principle upon which the present invention relies will be explained on the basis of a simplified structure referring to fig7 a cylinder 101a houses a resilient member 102a for carrying a load a supporting rod 103a connected to the load and other associated members the supporting rod 103a has an arm 105a projecting therefrom for operating a switch 104a an adjusting screw 106 is provided for imparting a predetermined initial load to the resilient member 102a in this arrangement when the load not shown suspended from the supporting rod 103a exceed a pressure po provided by the resilient body 102a the arm 105a comes to depress the switch 104a to open the later thereby to detect an overload to stop the hoisting for a load smaller than the pressure po the support provided by the resilient member can be regarded as a rigid support i e no shrinkage of the resilient member 102a takes place referring to fig8 showing a principle of conventional apparatus no initial tension is given to the resilient member 102b therefore the switch 104b is operated when the load applied to the resilient member 102b comes up to po i e when a strain u03b4 0 is caused in the resilient member 102b the resilient member 102b is compressed to shrink even for a load smaller than po this conventional arrangement also has a cylinder 101b supporting rod 103b and an arm 105b it will be understood from the foregoing description that in the arrangement of the present invention no.