A main body includes a front engaging part on a front side thereof. The front engaging part is engaged with an interior part of the vehicle. The main body is pivotable about the front engaging part. A first string includes a first connecting part connected to the main body rearward of the front engaging part and a first engaging part engaged with a pivotable back door. A second string includes a second connecting part connected to the main body rearward of the front engaging part and a second engaging part engaged with a vehicle main body. The first string is subjected to tension due to weight of the main body and the second string is loosened when the back door is opened. When the main body is moved to a direction that causes the first string to loosen in said state, the second string is subjected to tension.