This hydraulic oil tank, which is disposed inside a pod of a power generation system, stores hydraulic oil, wherein the hydraulic oil tank is provided with: a rubber bag for use during oil temperature increases, the rubber bag being attached to a tank wall surface from the inner side, having a first internal space communicated with the exterior through an opening passing through the wall surface, and partitioning the first internal space and the wall surface interior in an elastic manner; and a rubber bag for use during oil temperature decreases, the rubber bag being attached to the wall surface from the outer side, having a second internal space communicated with the wall surface interior through an opening passing through the wall surface, and partitioning the second internal space and the wall surface interior in an elastic manner.