PURPOSE: To allow optimizing the torque capacity of a friction engaging means even though the engine torque involves uncontinuous change, by furnishing an accumulator with a plurality of back pressure chambers having different pressure receiving area, and changing the back pressure chamber for supplying the oil pressure on the basis of adjusting operation of the engine torque.

CONSTITUTION: An accumulator 11 has a piston 15, wherein No.1 back pressure chamber 18 sealed with two seal rings 17 is formed at the periphery of a cylindrical part 16, inside whereof constitutes No.2 back pressure chamber 19 having a greater pressure receiving area than the No.1 back pressure chamber 18. A selector valve 12 changes over from the condition, in which No.1 drain port 29 is in communication with No.1 back pressure chamber port 30 and also an input port 31 is in communication with No.2 back pressure chamber port 32, to the condition in which the No.1 back pressure chamber port 30 is in communication with the input port 31 and also the No.2 back pressure chamber port 32 is in communication with No.2 drain port 33, or vice versa. The opened/ closed condition of a swirl control valve of engine E is informed from an electronic control unit 7 for engine to an electronic control unit 8 for automatic transmission, and the selector valve 12 is controlled.