PURPOSE: To prevent seizure in a transmission gear box for a pump driven by wind or water power via the transmission gear box by connecting a circulation pipe passage to circulate lubricating oil to the casing of the transmission gear box for cooling the lubricating oil in accordance with the heat generated in the transmission gear box.

CONSTITUTION: A drive shaft 3a of an oil pump 3 is connected to a shaft 1a of a windmill 1 via a transmission gear box 2, while an oil-pressure motor 40 is provided in the downstream of a throttle valve 6e in a discharge pipe passage branch part 5d of the oil pump 3. To a casing 2a of the transmission gear box 2, a circulation pipe passage 37 is connected for circulating the lubricating oil poured into the casing 2a. This pipe passage 37 is provided with a circulation pump 39 connected to a shaft 40a of the oil-pressure motor 40. The circulation pipe passage 37 in the downstream of this pump 39 is provided with a heat exchanger 41. The lubricating oil heated in the transmission gear box 2 is circulated in the pipe passage 37 by the operation of the circulation pump 39, and exchanges heat with the cold water supplied via a refrigerant pipe passage 8 in the heat exchanger 41 for cooling.