A welding method comprising: a welding step of performing a multilayer groove welding, characterised by a welding inspection step of performing a welding inspection method simultaneously with the welding step, the welding inspection method comprising: a transmission laser light irradiation step of generating transmission laser light (Ii) for generating an ultrasonic wave (Ui) and transmitting the transmission laser light (Ii) to a predetermined position (Pi) of an object to be inspected during welding operation for irradiation; a reception laser light irradiation step of generating reception laser light (Id) for detecting an ultrasonic wave (Ur) excited by the transmission laser light irradiation step and transmitting the reception laser light (Id) to a predetermined position (Pd) of the object to be inspected for irradiation; a light collection step of collecting the laser light (Ir) scattered and reflected at surface of the object to be inspected; an interference measurement step of performing interference measurement of the laser light (Ir) collected by the light collection step and obtaining an ultrasonic signal; and an analysis step of analyzing the ultrasonic signal obtained by the interference measurement step, wherein one of the transmission laser light (Ii) generated in the transmission laser light irradiation step and the reception laser light (Id) generated in the reception laser light irradiation step is irradiated onto a welded metal part, and the other of the transmission laser light (Ii) and the reception laser light (Ir) is irradiated on a surface of the object to be inspected not on the surface of the welded metal part. a welding step of performing a multilayer groove welding, characterised by a welding inspection step of performing a welding inspection method simultaneously with the welding step, the welding inspection method comprising: a transmission laser light irradiation step of generating transmission laser light (Ii) for generating an ultrasonic wave (Ui) and transmitting the transmission laser light (Ii) to a predetermined position (Pi) of an object to be inspected during welding operation for irradiation; a reception laser light irradiation step of generating reception laser light (Id) for detecting an ultrasonic wave (Ur) excited by the transmission laser light irradiation step and transmitting the reception laser light (Id) to a predetermined position (Pd) of the object to be inspected for irradiation; a light collection step of collecting the laser light (Ir) scattered and reflected at surface of the object to be inspected; an interference measurement step of performing interference measurement of the laser light (Ir) collected by the light collection step and obtaining an ultrasonic signal; and an analysis step of analyzing the ultrasonic signal obtained by the interference measurement step, wherein one of the transmission laser light (Ii) generated in the transmission laser light irradiation step and the reception laser light (Id) generated in the reception laser light irradiation step is irradiated onto a welded metal part, and the other of the transmission laser light (Ii) and the reception laser light (Ir) is irradiated on a surface of the object to be inspected not on the surface of the welded metal part. a transmission laser light irradiation step of generating transmission laser light (Ii) for generating an ultrasonic wave (Ui) and transmitting the transmission laser light (Ii) to a predetermined position (Pi) of an object to be inspected during welding operation for irradiation; a reception laser light irradiation step of generating reception laser light (Id) for detecting an ultrasonic wave (Ur) excited by the transmission laser light irradiation step and transmitting the reception laser light (Id) to a predetermined position (Pd) of the object to be inspected for irradiation; a light collection step of collecting the laser light (Ir) scattered and reflected at surface of the object to be inspected; an interference measurement step of performing interference measurement of the laser light (Ir) collected by the light collection step and obtaining an ultrasonic signal; and an analysis step of analyzing the ultrasonic signal obtained by the interference measurement step, wherein one of the transmission laser light (Ii) generated in the transmission laser light irradiation step and the reception laser light (Id) generated in the reception laser light irradiation step is irradiated onto a welded metal part, and the other of the transmission laser light (Ii) and the reception laser light (Ir) is irradiated on a surface of the object to be inspected not on the surface of the welded metal part.