Abstract: Portions of a thermoplastic synthetic resin panel (B) formed by extrusion and integrally having a first sheet (2a), a second sheet (2b) parallel to the first sheet (2a), and a plurality of ribs (3) integrally joining the first and second sheets (2a, 2b) are welded together by ultrasonic welding by applying an ultrasonic welding head (6) and a work support block (7) of an ultrasonic welding machine to the first sheet (2a) and the second sheet (2b), respectively, and compressing the portions of the thermoplastic synthetic resin panel (B) between the ultrasonic welding head (6) and the work support block (7).