To effectively protect various hydraulic devices, provided on a hydraulic circuit, by making constitution, wherein a pressure loss detecting means, for detecting the pressure loss of hydraulic fluid by a filter, and an output lowering means, for lowering the output of an hydraulic circuit, are provided to actuate the output lowering means based on a detected pressure loss value is exceeding a preset value.

In a control part 35, in the cases where a filter is in a blinding condition, and timing for maintenance is judged, or the contamination of hydraulic fluid is progressed, and timing for replacement is judged; corresponding anounciators 37, 38, and 39 inform these judgements to an operator, to urge maintenance, and also an engine rotation frequency is reduced to a low rotation frequency, thereby lowering the output of a hydraulic circuit. Consequently, oil pressure output, applied to the various kind hydraulic devices such as a hydraulic actuator, or a piping and a valve etc., provided on the hydraulic circuit, can be reduced, thereby effectively protecting these hydraulic devices.