An arc sensor (10) to be attached to an arc welding torch for detecting a welding spot is provided with an optical detecting system having an optical position detector (14), an objective lens (18), and a transparent objective window (20) arranged in front of the objective lens, and a plurality of arc light screening plates (22 and 24) made of an opaque material, and having a light transmitting apertures (22a and 24a), the plurality of arc light screening plates (22 and 24) being arranged in front of the transparent objective window (20) and spaced from one another for screening an arc light but permitting the detecting light beam to pass therethrough.