A low strength part 62, which is low in rigidity and on which stress concentrates, is formed along a portion demarcating the end edge part 61 of a flange 32b located at the inner part of a case, which faces the top end part of a fixed terminal 41. The evaluation is based on, for example, an attribute such as a direction of the road, an incline of the road or the like. A flexible rod having a first end and a second end, is attached to the tether attachment piece at its first end.