The present invention comprises: an opening/closing plate formed in a plate shape and having one side surface coming into contact with a moving passage part, a screw coupling part formed on the other side surface, and a first elastic member seating part on the outer circumference of the screw coupling part so as to allow one side surface of a first elastic member to be seated thereon; at least one shaft connecting bar provided on the other surface of the opening/closing plate, and having a contact protrusion part having a second elastic member seating part formed to allow the other side surface of the first elastic member to be seated thereon, and a connecting part formed in the contact protrusion part so that a connecting unit is inserted; the first elastic member provided between the first elastic member seating part and the second elastic member seating part so as to guide the elastic connection between the opening/closing plate and the shaft connecting bar; and the connecting unit inserted into the connecting part so as to be connected to the other side surface of the opening and closing plate.