A welding system has: a welding mechanism, a reception laser light source, a reception optical mechanism, an interferometer, a data recording/ analysis mechanism and a data recording/ analysis mechanism. The reception laser light source generates reception laser light so as to irradiate the object to be welded with the reception laser light for the purpose of detecting a reflected ultrasonic wave obtained as a result of reflection of a transmission ultrasonic wave. The reception optical mechanism transmits, during or after welding operation, the reception laser light generated from the reception laser light source to the surface of the object to be welded for irradiation while moving, together with the welding mechanism, relative to the object to be welded and collects laser light scattered reflected at the surface of the object to be welded.