A rubber plug holding device, comprising: a plurality of first clamp members located around a predetermined central line so as to form a space inner thereto in which a cylindrical rubber plug is allowed to be located; a first support member supporting the plurality of first clamp members; and a first actuator driving the plurality of first clamp members, wherein: the plurality of first clamp members include three or more first rubber plug contact portions (31e1) that are on the space inner thereto and are to be in contact with an outer circumferential surface of the rubber plug, the first support member supports the first clamp members such that the first rubber plug contact portions (31e1) get closer to, or farther from, the central line, characterized in that: the first support member includes a plurality of first pivoting shafts each extending parallel to the central line and respectively supporting the plurality of first clamp members such that said first clamp members are pivotable, and the first actuator drives the first clamp members such that the first clamp members pivot around the first pivoting shafts such that the first rubber plug contact portions (31e1) get closer to, or farther from, the central line. a plurality of first clamp members located around a predetermined central line so as to form a space inner thereto in which a cylindrical rubber plug is allowed to be located; a first support member supporting the plurality of first clamp members; and a first actuator driving the plurality of first clamp members, wherein: the plurality of first clamp members include three or more first rubber plug contact portions (31e1) that are on the space inner thereto and are to be in contact with an outer circumferential surface of the rubber plug, the first support member supports the first clamp members such that the first rubber plug contact portions (31e1) get closer to, or farther from, the central line, characterized in that: the first support member includes a plurality of first pivoting shafts each extending parallel to the central line and respectively supporting the plurality of first clamp members such that said first clamp members are pivotable, and the first actuator drives the first clamp members such that the first clamp members pivot around the first pivoting shafts such that the first rubber plug contact portions (31e1) get closer to, or farther from, the central line. the plurality of first clamp members include three or more first rubber plug contact portions (31e1) that are on the space inner thereto and are to be in contact with an outer circumferential surface of the rubber plug, the first support member supports the first clamp members such that the first rubber plug contact portions (31e1) get closer to, or farther from, the central line, characterized in that: the first support member includes a plurality of first pivoting shafts each extending parallel to the central line and respectively supporting the plurality of first clamp members such that said first clamp members are pivotable, and the first actuator drives the first clamp members such that the first clamp members pivot around the first pivoting shafts such that the first rubber plug contact portions (31e1) get closer to, or farther from, the central line. the first support member includes a plurality of first pivoting shafts each extending parallel to the central line and respectively supporting the plurality of first clamp members such that said first clamp members are pivotable, and the first actuator drives the first clamp members such that the first clamp members pivot around the first pivoting shafts such that the first rubber plug contact portions (31e1) get closer to, or farther from, the central line.