PURPOSE: To surely alarm ground subsidence of an outrigger jack contact ground for a mobile crane.

CONSTITUTION: A sinkage alarming device is provided with a rotating angle detecting means 6, an elevation/depression angle detecting means 7, a length detecting means 8, a load detecting means 9 detecting a load working on a telescopic boom, an extension quantity detecting mean 10 detecting extension quantity of each outrigger jack respectively, and a supporting force detecting means 11 detecting supporting force of each outrigger jack as detected supporting force. In addition, a computing means 12, which receives detection signals from respective detecting means and computes them so as to output an alarm signal, and an alarming device 13, which receives the alarm signal so as to be operated, are provided. The computing means 12 computes supporting force of each outrigger jack as computed supporting force, and also, compares the computed supporting force with the detected supporting force so as to generate the alarm signal when a deviation of the predetermined value or more is generated between them.