PURPOSE: To prevent the occurrence of an accident by bringing a compressor to rapid stop after a trouble occurs, by constituting a device such that the occurrence of a trouble at the compressor itself or in a fluid passage is detected based on a change in the ratio of the number of revolution between an external driving source and the compressor to shut off an input to the compressor.

CONSTITUTION: Sensors 10 and 11 for the number of revolution are attached to the front end part of a crank shaft 2, acting as the output rotary member of an external driving source, and an input shaft 5 working as the input rotary member of a compressor 4. Output signals therefrom are inputted to a comparing circuit 12 to compare them with each other and the results are inputted to a controller 13. If a difference, exceeding a given value, in the ratio of the number of revolution of an engine 1 to that of the compressor 4 is produced for given consecutive seconds, for example, about 0.6sec, it is decided that a trouble occurs to the compressor 4, and a signal, by means of which an input to the compressor 4 is shut off, is transmitted to an electromagnetic clutch 6 from the controller 13. This permits complete prevention of the occurrence of an accident resulting from continuous operation without finding out a trouble after it occurs.