A piston operating a first clutch is disposed between a first drum accommodating the first clutch and a second drum accommodating a second clutch, and an oil reservoir storing lubricating oil that has lubricated the first clutch is provided on an inner cylindrical surface of a skirt portion of the piston. Barriers having different inner diameters are provided to the axial front and rear of the oil reservoir. When the second clutch is disengaged such that the amount of lubricating oil supplied to the oil reservoir increases, the lubricating oil overflows the barrier having the smaller inner diameter, and thus flows to the second clutch.