The present invention is directed to provide an improved structure of a sash of the side door for automobiles, wherein rigidity of the door sash, hence rigidity of the side door as a whole, is augmented without increase in weight of the door sash, and without causing hindrance to the surrounding view. Such structure is realized by the box-shaped transverse sectional part (45) provided over the entire length of the door sash (4, 4A, 4B, 4C), of which the size (w) in the breadthwise direction of the vehicle is gradually changed, thereby progressively reducing thickness (W) of the door sash (4, 4A, 4B, 4C) in the breadthwise direction of the vehicle.