An apparatus for welding monitoring for detecting a caught-in defect caused by an incorporated foreign substance, wherein the apparatus monitors a welding state of a V-convergence region in which a strip-shaped metal sheet is converged in a V-shape, when the metal sheet is cylindrically formed while being conveyed, and both side edges (1a, 1b) of the metal sheet are heated and melted in a manner of being butted each other while being converged in the V-shape, such that an electric resistance welded steel pipe is manufactured, the apparatus comprising: an image capturing unit that captures RGB images of a region including the V-convergence region in time series; and being characterized by an image processing unit that: extracts a red image and a blue image from the RGB images; performs inverted binarization and labeling of the red image to detect a V-convergence point in the red image, with respect to the red image, at which both the side edges (1a, 1b) of the metal sheet are converged based on the images captured in time series; performs binarization and labeling of the blue image to detect a high-luminance portion in the blue image, with respect to the blue image, corresponding to the presence and a position of arcing; and compares a position of the V-convergence point and the position of the high-luminance portion to determine whether the arcing corresponds to an irregular arcing, which is arcing occurring at the V-convergence point or on an upstream side of the V-convergence point, to specify the presence of the defect. an image capturing unit that captures RGB images of a region including the V-convergence region in time series; and being characterized by an image processing unit that: extracts a red image and a blue image from the RGB images; performs inverted binarization and labeling of the red image to detect a V-convergence point in the red image, with respect to the red image, at which both the side edges (1a, 1b) of the metal sheet are converged based on the images captured in time series; performs binarization and labeling of the blue image to detect a high-luminance portion in the blue image, with respect to the blue image, corresponding to the presence and a position of arcing; and compares a position of the V-convergence point and the position of the high-luminance portion to determine whether the arcing corresponds to an irregular arcing, which is arcing occurring at the V-convergence point or on an upstream side of the V-convergence point, to specify the presence of the defect. extracts a red image and a blue image from the RGB images; performs inverted binarization and labeling of the red image to detect a V-convergence point in the red image, with respect to the red image, at which both the side edges (1a, 1b) of the metal sheet are converged based on the images captured in time series; performs binarization and labeling of the blue image to detect a high-luminance portion in the blue image, with respect to the blue image, corresponding to the presence and a position of arcing; and compares a position of the V-convergence point and the position of the high-luminance portion to determine whether the arcing corresponds to an irregular arcing, which is arcing occurring at the V-convergence point or on an upstream side of the V-convergence point, to specify the presence of the defect.