PURPOSE: To provide a hydraulic oil purifying device where its installation cost is low and a narrow space is good enough for its installation and a filter is easily replaced.

CONSTITUTION: A hydraulic oil purifying device S is installed in a hydraulic circuit consisting of an oil tank 1, a hydraulic pump 4, a hydraulic load circuit R, etc. The hydraulic oil purifying device S is constituted of a jet pump 11 to which hydraulic oil delivered from a hydraulic pump 4 is fed and a filter device 20 which is inserted into a suction line 25 where hydraulic oil in the oil tank 1 is sucked by the jet pump 11 and forcibly fed to the oil tank. The filter device 20 is located higher than the oil tank 1. Therefore, since hydraulic oil in the oil tank 1 is circulated by the jet energy of hydraulic oil delivered from the hydraulic pump 4, a driving source for purification such as a motor is not particularly needed, causing installation cost to be low and the device is not limited in installation space.