PURPOSE:To smooth discharge of lubricating oil so as to restrain drag torque by forming, in one circle on the inner circumferential surface of a case for containing a rotary element, a connecting groove for connecting spline grooves to each other formed on the inner circumferential surface. CONSTITUTION:In the inner circumferential surface of a case 20, a connecting grooves 24 extending in the axial direction of a rotary element 21 and with a multiplicity of spline grooves 23 arranged around the axis for connecting the spline grooves 23 is formed on at least one circle of an inner circumferential surface 22. According to the rotation of the rotary element 21, lubricating oil supplied to a driven plate 25 and a drive plate frictionally engaged with the driven plate 25 is used for lubrication of both plates, before reaching the spline grooves 23, and then is introduced into a discharge passage 28 under guidance of the connecting groove 24. The lubricating oil intruded into a space between the driven plate 25 and the inner circumferential surface 22 drops in the spline grooves 23 where discharge of the oil has been completed, and then, is introduced to the discharge passage 28 in the same manner as described above. Therefore, there is no residual lubricating oil so that drag torque generated between the driven plate 25 and the drive plate can be restrained.
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