PURPOSE: To superpose the rail of a ventilator and the frame of a fixed window intermediate rail or the like on top of each other along interior and exterior directions, when the ventilator is opened, thereby increasing an open area and improving appearance.

CONSTITUTION: The door positioning rail side engagement piece 18 of a ventilator 10, and the engagement piece 27 of a fixed window 20 coupled thereto, are laid at a position nearer the fixed window 20 than a check rail side engagement piece 19. As a result, even when the ventilator 10 is opened, the engagement piece 19 does not collide with the engagement piece 27 of the window 20, and a rail 15 can be superposed on the intermediate rail 21 of the window 20 in interior and exterior directions, when the ventilator 10 is opened, thereby increasing an open area and improving appearance.