Disclosed are shoulder belt sliding-down preventing method, and a related preventing tool, shoulder belt, and bag. The belt-shaped neck hanging element is connected to a belt at positions separated from each other via connecting members. A loop-shaped space is thereby formed between the belt body and the neck-hanging element. The head passes through this space, and the neck hanging element contacts the side of the neck. In operation, the connecting member is removed and held with one hand, the belt body is hung on the shoulder, the connecting member is passed around the back of the neck, the neck hanging element is hung around the neck, and the connecting member is engaged again with the belt body. The belt body of the shoulder bag can hang on the shoulder and the neck hanging element can be wound around the neck, thereby preventing the belt body from sliding down the shoulder.