PURPOSE: To detect the occurrence of abnormal external oil leakage at an early time by detecting the quantity of external oil leakage from the time history of level decrease within a preset unit time.

CONSTITUTION: The electric signal quantity corresponding to the oil quantity portion which cahanges oil levels going into or out from a hydraulic tank along with operation of a main large capacity cylinder 1 is set with an addition-subtraction setting part 10 and only at the instruction from a cylinder operating circuit 8 to a solenoid valve 2, the foregoing oil level electric signal quantity is subjected to addition or subtraction by an adder-subtractor 9. For the oil levels fluctuating with frequent operations of plural small capacity cylinders and oil supply of an accumulator 4 by an oil pump 3, the oil quantity corresponding to the static state in the hydraulic system is detected by an average operating part 11 which averages these and an average time setting part 12 which sets the time to be averaged.