Abstract: A marking device that targets a fastened member and a fastening member (5,6), the marking device comprising; a housing that covers the fastening member (5, 6); and a marking member that is coupled to the housing via a support shaft (30) and is swingable toward the fastening member (5, 6), characterized in that when a vector having, as a start point, a marked corner part (MC1) and having the swing center (31) as an end point at is a first vector (V1a), and a vector having the marked corner part (MC1) as a start point, and is directed in a direction orthogonal to a central axis (C) of the fastening member (5) and away from the fastening member (5, 6) is a second vector (V2a), an angle (?1) formed by these two vectors (V1a, V2a) is equal to or greater than 90 degrees and less than 180 degrees.