To accurately diagnose the welded state of a welded layer formed by mutually abutting the end surfaces of two synthetic resin pipes to weld and join them.

Ultrasonic probes 18, 20 are arranged on both sides of the welded layer 16 on the outer peripheral surfaces of two pipes 12, 14 and allow ultrasonic pulses to be alternately incident on the welded layer 16 within the wall thickness of the pipes 12, 14 at a predetermined incident angle to receive the reflected waves from the interiors of the wall thicknesses of the pipes 12, 14 corresponding to respective incident waves. Then, the time up to the reception of the interfacial reflected waves of the welded layer 16 contained in the respective received reflected waves is compared with the set time to a gate set in order to pass the respective interfacial waves in such a case that the welded layer 16 is formed into width capable of judging that a welded state is good to judge the quality of the welded state.