hole 18 in this state and the screw grommet is reliably engaged with the first panel 17 subsequently a second panel 19 is held with its mounting hole 20 aligned with the screw insertion hole 2 of the flange 1 and a shaft portion 21a of a screw 21 successively inserted through the holes 20 and 2 and into the screw guide channel 10 and then the screw 21 is rotated as a result a thread 21b formed upon the outer periphery of the screw shaft 21a taps the wall surface of the screw guide channel 10 that is the inner surfaces of the opposite side walls 8a of the constricted portion 8 so as to form a helical thread corresponding to the thread 21b upon screw shaft 21a within the wall surfaces of the screw guide channel 10 as the screw shaft 21a is advanced within the screw guide channel 10 in this way the oppositely disposed hill like portions 12 are expanded outwardly away from each other together with their respective side walls 8a of the constricted portion 8 these hill like portions 12 strongly engage the first panel 17 whereby the first panel 17 is secured to the screw grommet as shown in fig9 that is the two panels.