A cabin-interior upright wall of a door inner panel is configured by a first panel member alone. An extension wall extends toward a door outer panel from an end of the cabin-interior upright wall. A lower portion of a front upright wall, which is part of the extension wall, is segmented in a door thickness direction. The extension wall is configured such that a first extension portion of a first panel member is joined to a second extension portion of a second panel member in a state overlapped with the second extension portion.