Welded state monitoring system and welded state monitoring method

Provided are a welded state monitoring system and a welded state monitoring method. This welded state monitoring system is used for plasma shielded electric resistance welding, in which electric resistance welding is performed while a weld zone (3) of a steel sheet (1) is irradiated with plasma, and is provided with: a plasma irradiation device (2) which irradiates the weld zone with the plasma; a first image capturing device which captures an image of the weld zone from above and has an image sensor capable of detecting light with a wavelength of 850 nm or more; a first wavelength region limiting device which limits light incident on the first image capturing device to the wavelength region of 850 nm or more; and an image processing device which subjects the image captured by the first image capturing device to image processing and analyzes the welded state of the weld zone, thereby being able to analyze the welded state without being affected by the plasma.