To provide a hydraulic operation device capable of suppressing a hydraulic fluid stored in an open-type hydraulic oil tank from generating a jelly-like foreign substance and lengthening the replacement period of the hydraulic oil.

The open-type hydraulic oil tank 13 stores the hydraulic oil 12 that is supplied to a hydraulic drive section 11, has a ventilation port 16 that intakes and exhausts the air accompanying changes which arise in the hydraulic oil 12 of the hydraulic oil tank 13 when the hydraulic oil 12 is supplied to the hydraulic drive section 11 from the hydraulic oil tank 13 by a pump 14 or the hydraulic oil 12 is returned to the hydraulic oil tank 13 from the hydraulic drive section 11, a cylinder 17 attached to the ventilation port 16 of the hydraulic oil tank 13 keeps constant an internal pressure in the hydraulic oil tank 13 by intaking and exhausting the air if the amount of the hydraulic oil 12 that is supplied to the hydraulic drive section 11 is fluctuated, and a piston 18 provided inside the cylinder 17 prevents the hydraulic oil 12 from contacting the air and is reciprocated in accordance with changes made in the oil level of the hydraulic oil 12 inside the cylinder 17.