A clamper comprising: a clamp mechanism configured to insert an insertion portion into a tube member and to clamp the tube member; and a lifting and lowering mechanism configured to lift and to lower the clamp mechanism in the insertion direction of the insertion portion, wherein the clamp mechanism includes a clamp body which includes the insertion portion,the insertion portion being provided with a slit, a cotter is inserted in the slit and is configured to protrude from the insertion portion so as to come into friction-contact with the tube member and to be movable in a reciprocating manner in the insertion direction of the insertion portion, a piston rod configured to press the cotter so that the cotter protrudes from the insertion portion when being pulled toward the opposite side to the insertion direction of the insertion portion, a rod cylinder which is integrated with the clamp body and configured to pull the piston rod, and a cotter diameter decreasing structure configured to decrease the diameter of the cotter, characterized in that the cotter diameter decreasing structure includes a tapered surface which is formed on the cotter and is disposed toward a front end portion of the clamp body inside the slit, and an elastic member which is inserted into the clamp body and is supported by the clamp body so as to be biased to a rear end portion of the cotter wherein by the biasing force of the elastic member, the cotter presses the tapered surface against a wall surface near a front end portion of the slit, and in that the cotter diameter decreasing structure is configured to bias the tapered surface to the clamp body, and to slide the cotter along the tapered surface by the biasing force from the elastic member to decrease the diameter of the cotter. a clamp mechanism configured to insert an insertion portion into a tube member and to clamp the tube member; and a lifting and lowering mechanism configured to lift and to lower the clamp mechanism in the insertion direction of the insertion portion, wherein the clamp mechanism includes a clamp body which includes the insertion portion,the insertion portion being provided with a slit, a cotter is inserted in the slit and is configured to protrude from the insertion portion so as to come into friction-contact with the tube member and to be movable in a reciprocating manner in the insertion direction of the insertion portion, a piston rod configured to press the cotter so that the cotter protrudes from the insertion portion when being pulled toward the opposite side to the insertion direction of the insertion portion, a rod cylinder which is integrated with the clamp body and configured to pull the piston rod, and a cotter diameter decreasing structure configured to decrease the diameter of the cotter, a clamp body which includes the insertion portion,the insertion portion being provided with a slit, a cotter is inserted in the slit and is configured to protrude from the insertion portion so as to come into friction-contact with the tube member and to be movable in a reciprocating manner in the insertion direction of the insertion portion, a piston rod configured to press the cotter so that the cotter protrudes from the insertion portion when being pulled toward the opposite side to the insertion direction of the insertion portion, a rod cylinder which is integrated with the clamp body and configured to pull the piston rod, and a cotter diameter decreasing structure configured to decrease the diameter of the cotter, characterized in that the cotter diameter decreasing structure includes a tapered surface which is formed on the cotter and is disposed toward a front end portion of the clamp body inside the slit, and an elastic member which is inserted into the clamp body and is supported by the clamp body so as to be biased to a rear end portion of the cotter wherein by the biasing force of the elastic member, the cotter presses the tapered surface against a wall surface near a front end portion of the slit, and a tapered surface which is formed on the cotter and is disposed toward a front end portion of the clamp body inside the slit, and an elastic member which is inserted into the clamp body and is supported by the clamp body so as to be biased to a rear end portion of the cotter wherein by the biasing force of the elastic member, the cotter presses the tapered surface against a wall surface near a front end portion of the slit, and in that the cotter diameter decreasing structure is configured to bias the tapered surface to the clamp body, and to slide the cotter along the tapered surface by the biasing force from the elastic member to decrease the diameter of the cotter.