An oil tank (28) is disposed in a nacelle (7) supported on a tower (14) of a wind turbine generator (100), for storing oil used in an oil using unit (4) included in the wind turbine generator (100). The oil tank (28) at least includes: a first oil chamber (48) and a second oil chamber (50) for storing the oil; a partition wall (52) which separates the first oil chamber (48) and the second oil chamber (50); and an oil heater (56) for heating the oil in the oil tank. The partition wall (52) includes at least two openings (54a, 54b) through which the first oil chamber (48) is in communication with the second oil chamber (50) so that the oil is movable between the first oil chamber (48) and the second oil chamber (50).