The present invention provides an ultrasonic welding structure increased in length, width and height as desired in which disassembly or classification is unnecessary by forming joints of the same material when recycling an extended container, metal components are not mixed in the plastic as foreign matters, and the strength of joined portions of plastic-made members is increased. In the ultrasonic welding structure, a joined portion J is formed by butting the plastic-made members to each other, and an ultrasonic horn 2 is pressed against the joined portion J to perform the ultrasonic welding. A plurality of uneven marks 9 are formed in the joined portion J. |