A method for monitoring welding of an electric resistance welded steel pipe, comprising: an image acquisition step in which, when an electric resistance welded steel pipe is welded, a V-convergence portion including an edge detection area and a V-convergence point, and a welding portion including a position where a molten steel starts to be discharged from an inside of a wall thickness and a bead, are imaged to obtain a captured image; a determination information acquisition step in which the captured image is image-processed, and a molten length from the V-convergence point to the position where the molten steel starts to be discharged is acquired as a determination information wherein the molten length is obtained by extracting one of red component, green component, and blue component, from the captured image, detecting a pair of approximation straight lines of butt end surfaces (e1, e2), detecting an intersection between the pair of approximate straight lines as the V-convergence point, setting an area to be re-binarized downstream from the V-convergence point, re-binarizing the area to obtain a re-binarization area, extracting a high brightness area by a labeling process of labeling the re-binarization area, extracting blobs in contact with a left end of the re-binarization area, to obtain the position where the molten steel starts to be discharged, the blobs being white areas detected as a mass in the labeling process, and calculating a length from the V-convergence point to the position where the molten steel starts to be discharged, to obtain the molten length, wherein the position where the molten steel starts to be discharged is defined by the most downstream coordinates of the blobs; and a determining step in which it is determined whether a cold welding has occurred by comparing the determination information with a determination threshold value set in advance for each wall thickness of the electric resistance welded steel pipe. an image acquisition step in which, when an electric resistance welded steel pipe is welded, a V-convergence portion including an edge detection area and a V-convergence point, and a welding portion including a position where a molten steel starts to be discharged from an inside of a wall thickness and a bead, are imaged to obtain a captured image; a determination information acquisition step in which the captured image is image-processed, and a molten length from the V-convergence point to the position where the molten steel starts to be discharged is acquired as a determination information wherein the molten length is obtained by extracting one of red component, green component, and blue component, from the captured image, detecting a pair of approximation straight lines of butt end surfaces (e1, e2), detecting an intersection between the pair of approximate straight lines as the V-convergence point, setting an area to be re-binarized downstream from the V-convergence point, re-binarizing the area to obtain a re-binarization area, extracting a high brightness area by a labeling process of labeling the re-binarization area, extracting blobs in contact with a left end of the re-binarization area, to obtain the position where the molten steel starts to be discharged, the blobs being white areas detected as a mass in the labeling process, and calculating a length from the V-convergence point to the position where the molten steel starts to be discharged, to obtain the molten length, wherein the position where the molten steel starts to be discharged is defined by the most downstream coordinates of the blobs; and a determining step in which it is determined whether a cold welding has occurred by comparing the determination information with a determination threshold value set in advance for each wall thickness of the electric resistance welded steel pipe.