The fusion device for fusing a synovial joint of a human or animal patient, in particular a human facet joint, finger joint or toe joint, includes two pin-shaped anchorage portions (1) and arranged therebetween a stabilization portion (2). The anchorage portions (1) include a thermoplastic material which is liquefiable by mechanical vibration. The stabilization portion (2) preferably has a surface which is equipped for enhancing osseointegration. The anchorage portions (1) have a greater thickness (T1) and a greater depth (D) than the stabilization portion (2). Then the fusion device is pushed between the articular surfaces and mechanical vibration, in particular ultrasonic vibration, is applied to the proximal face (4) of the fusion device. Thereby the liquefiable material is liquefied where in contact with the bone tissue and penetrates into the bone tissue, where after re-solidification it constitutes a positive fit connection between the fusion device and the bone tissue.