This hydraulic oil tank, which is disposed inside a pod (101) of a power generation system (100), stores hydraulic oil, wherein the hydraulic oil tank is provided with: a rubber bag (12) for use during oil temperature increases, the rubber bag (12) being attached to a tank wall surface (11) from the inner side, having a first internal space communicated with the exterior through an opening (12-1)passing through the wall surface (11), and partitioning the first internal space and the wall surface (11) interior in an elastic manner; and a rubber bag (13) for use during oil temperature decreases, the rubber bag (13) being attached to the wall surface (11) from the outer side, having a second internal space communicated with the wall surface (11) interior through an opening (13-1) passing through the wall surface (11), and partitioning the second internal space and the wall surface (11) interior in an elastic manner.