A stiffening structure for an automotive outer panel of the present invention has an outer-panel side member fixed to the automotive outer panel. Further, the stiffening structure has an annexed member adhered onto an inner surface of the automotive outer panel while interposing an adhesive therebetween at least one end of the annexed member is joined to the outer-panel side member. Furthermore, the stiffening structure has a position adjuster provided on a joint portion of the annexed member and the outer-panel side member. The position adjuster makes the annexed member movable in a direction of approaching the automotive outer panel or moving away therefrom.