In an automatic transmission, a lip portion of an oil seal can be prevented from riding up reliably when an impeller hub is inserted into the oil seal so as to be attached thereto, even when the lip portion is provided with a tightening margin. The impeller hub is attached to the oil seal from the lip portion side of the oil seal , with an end portion thereof in which a notch is formed. The impeller hub has a first chamfer portion formed in a bottom portion outer peripheral edge portion of the notch as an inclined surface that decreases radially in an axial direction of the impeller hub toward the end portion of the impeller hub, and an axial distance of the first chamfer portion from the bottom portion outer peripheral edge portion increases from both end sides of a circumferential direction of the notch toward a circumferential direction central portion of the notch.