To surely detect various welding state such as a state of arc generation, a state of molten pool and a shape of bead after welding.

The apparatus comprises two imaging means 10A, 10B to image a circumference of arc weld, a first band-pass filter 3 which is installed in an imaging range of one of the imaging means 10A and has a band-pass property in an infrared light wavelength region, a second band-pass filter 4 which is installed in an imaging range of the imaging means 10B and has a band-pass property in an ultraviolet light wavelength region, an illuminating means 5 having an emission spectrum of ultraviolet light wavelength region, an image composing means 6 to compose images taken by both imaging means 10A, 10B and an image display means 8 to display the image composed by the image composing means 6.