To accurately measure spatter generation and its behavior by using an image processing means.

An image of an arc generation position and its periphery is picked up into a plurality of frame images during arc welding. Each frame image is binarized or multi-valued to extract one or a plurality of isolated ranges of high luminance and detect positional information on each isolated region so extracted. On the basis of the positional information of each detected isolated region, presence or absence of continuity is judged for the isolated regions among the continuous frame images. A series of isolated region groups judged as having presence of continuity is recognized as one spatter generated with arc welding.