To provide a laser welding zone quality-monitoring method which enables surely to judge whether a blow hole or a pit shaped opening defect is generated or not during welding.

The luminous strength of plasma plume on the reverse side of a welding zone being He gas shielded is measured, and whether the measured strength becomes more than twice of the luminous strength at the time of steady welding or not is monitored, so that whether either one or both of a blow hole and a pit opening defect is generated or not is judged. Further, whether either or both of the blow hole and the pit opening defect is generated or not is judged by measuring the luminous strength of a specific wavelength light of nitrogen in the plasma plume light on the reverse side of the welding zone shielded with either gas of He gas, Ar gas, or mixed gas of Ar and He, and monitoring whether the measured strength is abnormally changed or not. The most preferable wavelength is 411.0 nm or 568.0 nm.