A difference processing circuit receives input of welding images taken by a CCD camera in real time and then differentiates power monitoring images recorded in a memory from welding images to obtain difference processing reflected light images. Due to the difference processing performed by a difference processing circuit, foreign matter images included in welding images and power monitoring images are eliminated and the difference processing reflected light images that are obtained do not include any foreign matter images. A quality judgment device uses the difference processing reflected light images obtained by a difference processing circuit and conducts a quality judgment. As described above, since difference processing reflected light images do not include any foreign matter images, a quality judgment device does not receive any influence from the foreign matter that is adhered to the protective glass and can conduct an excellent evaluation of the welding quality.