Abstract: A fixing structure of sashes of a motor vehicle door comprises a main door sash defining an upper and front contour of the door, a middle door sash extending below and along the main door sash, and a coupling reinforcing member securely coupling the leading ends of the two sashes. The coupling reinforcing member is bolted to a window shoulder reinforcing members on outer and inner door panels. A lower end portion of the coupling reinforcing member is coupled to the inner door panel via an adjusting bolt to make the lower end portion of the coupling reinforcing member adjustingly displaceable toward and away from the inner door panel. By adjusting the adjusting bolt, the coupling reinforcing member is angularly moved whereby the windowpane attaching plane of the middle door sash can be finely adjusted so as to match the windowpane attaching plane of the main door sash.