A spot welding system including a welding gun with an openable/closable electrode pair; and a dressing determination device determining a quality of a shaping process applied to an electrode of the electrode pair. The dressing determination device includes a visual sensor obtaining an image of the electrode, identifying a base and a tip of the electrode on the image and measuring a distance between the base and the tip on the image, by image processing; a visual sensor control section allowing the visual sensor to measure the distance immediately before and after the shaping process is performed; a cut amount calculation section calculating a difference between the distances measured by the visual sensor immediately before and after the shaping process is performed, as a cut amount of the electrode; and a cut amount judgment section judging whether the calculated cut amount is within a predetermined acceptable range.