To provide a hydraulic device cooling a storage hydraulic oil in an oil tank by a simple structure with reduced quantity of parts.

In the oil tank 1, a recessed part 3 cut off from the inside of the oil tank 1 and opened to the outside is formed by recessing from an upper plate 2 side and a low part 4 positioned just below the recessed part 3, isolated from the outside, and filled up with the storage hydraulic oil to the uppermost part is formed in the oil tank 1. A motor 6 is disposed to face the bottom face 3A of the recessed part 3 so that cooling air F by the fan 6A of the motor 6 can be applied to the bottom face 3A of the recessed part 3.