A pin 6 provided at a horn 5 for ultrasonic welding is press-fitted to the depth reaching the light guide plate 1 breaking through the various kind of sheets 2 and 3 from surface side thereof. Another embodiment of the invention is that pins 25 constructed so as to break through various kind of sheets 22 and 23 on the surface of the plate are provided on the light guide plate 21, and the various kind of sheets 22 and 23 are fixed on the light guide plate 1 by distorting by caulking using ultrasonic vibration of the tip end of the pin.