To continuously remove the foreign matters contained in a hydraulic oil without maintenance in an automatic transmission which is mounted in a vehicle.
This automatic transmission 10 includes an oil flow passage 14 through which the hydraulic oil flows and a filter 16 for removing the foreign matters contained in the hydraulic oil. The filter 16 is diagonally installed in the oil flow passage 14 so that the upper part thereof is positioned on the upstream side and the downstream part thereof is positioned on the downstream side. A reservoir tank 18 is installed above the filter 16 and further on the downstream side than the filter 16. A foreign matter accumulating part 20 for accumulating the foreign matters is provided below the filter 16 and further on the upstream side than the filter 16. The hydraulic oil falling from the reservoir tank 18 to the oil flow passage 14 by the gravity flows from the downstream side toward the upstream side of the filter 16, and the foreign matters adhered to the filter 16 fall to the foreign matter accumulating part 20. Consequently, the clogging of the filter 16 can be eliminated without maintenance.