Ultrasonic joining of polytetrafluoroethylene (PTFE; TeflonTM) sheets using a lateral face of a rod vibrating longitudinally at 19 kHz is studied. We joined two PTFE sheets 0.3–0.8 mm thick using a 19 kHz longitudinal-mode joining system having a 10 mm×80 mm joining area. The joining tool was a straight bar vibrating longitudinally at 19 kHz. The lateral face of the bar could rub the PTFE face strongly. The temperature rise caused by the rubbing was high enough to enable joining. Joining was accomplished in 3–5 s. The maximum joint strength of sheets 0.5 mm thick was 2.4 MPa under the condition that static pressure was 6 MPa and joining time was 10 s.