A welding system comprising: a welding mechanism that is configured to weld an object to be welded while moving along a welding line relative to the object to be welded; a transmission laser light source that is configured to generate transmission laser light (Ii); a transmission optical mechanism that is configured to transmit, during or after welding operation, the transmission laser light (Ii) generated from the transmission laser light source to surface of the object to be welded for irradiation while moving, together with the welding mechanism, relative to the object to be welded so as to generate a transmission ultrasonic wave (U, Ui); a reception laser light source that is configured to generate reception laser light (Id) for the purpose of detecting a reflected ultrasonic wave (U, Ur) obtained as a result of reflection of the transmission ultrasonic wave (U, Ui); a reception optical mechanism that is configured to transmit, 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 (Ir) scattered/ reflected at the surface of the object to be welded; an interferometer that is configured to perform interference measurement of the scattered/ reflected laser light (Ir); and a data recording/ analysis mechanism that is configured to measure and analyze an ultrasonic signal obtained by the interferometer, characterized in that the welding system further comprises a surface modification mechanism (14a, 14b) that is configured to enhance sensitivity to at least one of the ultrasonic signal generated on the surface of the object to be welded onto which the transmission laser light (Ii) is irradiated and ultrasonic signal generated on the surface of the object to be welded onto which the reception laser light (Id) is irradiated, and the surface modification mechanism is configured to apply a high temperature resistant material onto at least part of the surface of the object to be welded onto to which the transmission laser light (Ii) is irradiated and part of the surface of the object to be welded onto which the reception laser light (Id) is irradiated. a welding mechanism that is configured to weld an object to be welded while moving along a welding line relative to the object to be welded; a transmission laser light source that is configured to generate transmission laser light (Ii); a transmission optical mechanism that is configured to transmit, during or after welding operation, the transmission laser light (Ii) generated from the transmission laser light source to surface of the object to be welded for irradiation while moving, together with the welding mechanism, relative to the object to be welded so as to generate a transmission ultrasonic wave (U, Ui); a reception laser light source that is configured to generate reception laser light (Id) for the purpose of detecting a reflected ultrasonic wave (U, Ur) obtained as a result of reflection of the transmission ultrasonic wave (U, Ui); a reception optical mechanism that is configured to transmit, 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 (Ir) scattered/ reflected at the surface of the object to be welded; an interferometer that is configured to perform interference measurement of the scattered/ reflected laser light (Ir); and a data recording/ analysis mechanism that is configured to measure and analyze an ultrasonic signal obtained by the interferometer, characterized in that the welding system further comprises a surface modification mechanism (14a, 14b) that is configured to enhance sensitivity to at least one of the ultrasonic signal generated on the surface of the object to be welded onto which the transmission laser light (Ii) is irradiated and ultrasonic signal generated on the surface of the object to be welded onto which the reception laser light (Id) is irradiated, and the surface modification mechanism is configured to apply a high temperature resistant material onto at least part of the surface of the object to be welded onto to which the transmission laser light (Ii) is irradiated and part of the surface of the object to be welded onto which the reception laser light (Id) is irradiated.