Abstract: A blood bag system includes one clamp mounted over a second tube and a third tube. In a first insertion hole of a main body of the clamp, a portion of a first blocking portion excluding a portion connected with a first opening portion is closed by the main body. The main body can be deformed from a first state to a second state. In the first state, the third tube having been moved into a second blocking portion of a second insertion hole is restrained by the second blocking portion. In the second state, the third tube can be moved from the second blocking portion to a position that is other than a second opening portion and in which a third flow path of the third tube is reopened.