To prevent an increase in slippage between a ring and a friction wheel resulting from a local rise of oil temperature due to insufficient stirring in an oil reservoir.

An oil circulation device 51 is connected to an oil reservoir 50. The oil circulation device 51 discharges the oil sucked in from a suction port 51a placed on a bottom surface side Fd in a depth direction in a deep part of the oil reservoir 50 and then cooled down by an oil cooler 53 from a discharge port 51b placed on an oil level 50a side with respect to the suction port. Thereby, a circulating flow is generated in the oil reservoir 50.