A module for incorporation into a door includes a base panel made of resin, a window motor attached to the base panel, a pinion gear, which is rotated by operation of the window motor, a carrier for retaining a window glass, and a power transmission arm, which is rotatably supported by the base panel. The power transmission arm includes a gear portion, which is engaged with the pinion gear, an arm portion, which has a distal end to be coupled to the carrier, and a coupler, which couples the gear portion to the arm portion. The coupler is formed of resin material having the same coefficient of linear expansion as the base panel. As a result, the engagement between the pinion gear and the gear portion is always reliably maintained regardless of temperature change.