PURPOSE: To accurately detect whether the operation detection switch of an operation means is normally operated or is at fault by simple constitution.

CONSTITUTION: When an operation is performed in the expansion direction and the reduction direction of the respective cylinders of a boom, an arm and a bucket, pressure switches 30a-30f are turned ON. When set time t2 by a second timer 35 elapses after it is detected that one of them is turned ON in a motion detection circuit 34, when all the pressure switches 30a-30f are turned ON respectively even once, flip-flop circuits 33a 33f connected through comparators 31a-31f are turned to a set state. Thus, signals from the flip-flop circuits 33a-33f are fetched to a fault judgement circuit 36, a normal operation state is judged when all the flip-flop circuits 33a-33f are turned to the set state and a fault state is judged when one of them is turned to a reset state in the fault judgement circuit 36.