A method of forming a seat for a fixture for attaching a window to a vehicle in one piece by forming a cavity between a shaping mold and the window and injecting a resinous material for forming the seat into the cavity, wherein a recess is formed at a side surface of the resin molding die; a window is placed on the mold so as to arrange the recess at a position corresponding to an inner peripheral region which is apart by a predetermined distance from the edge of the window thereby forming the cavity in association with the recess and the surface of the window; and the resinous material is injected into the cavity, followed by curing the resinous material, whereby the seat is formed in one piece on the window at the position inside by a predetermined distance from the edge of the window.