A clamp for connecting a first tubular member having a circular flange formed in an end portion thereof with a second tubular member having a circular flange formed in an end portion thereof, in a state where the circular flanges (511, 512) face each other and by covering outer peripheries of the circular flanges (511, 512) with the clamp, the clamp comprising: a support member; a first arm having an end pivotally supported by the support member; a second arm disposed to face the first arm, having an end pivotally supported by the support member; a first concave portion formed on the first arm on a side facing the second arm and formed in a curved shape corresponding to the circular flanges (511, 512) to fit a first region along the outer peripheries of the circular flanges (511, 512); a second concave portion formed on the second arm on a side facing the first arm and formed in a curved shape corresponding to the circular flanges (511, 512) to fit a second region along the outer peripheries of the circular flanges (511, 512); a connection member connecting the first arm and the second arm; characterized in that the clamp further comprises: a first guide portion formed close to the end of the first arm and engaged with a first end of the connection member through a first engaging member; and a second guide portion formed close to the end of the second arm and engaged with a second end of the connection member through a second engaging member; a third concave portion formed on the connection member on an opposite side to the support member side, to fit a third region along the outer peripheries of the circular flanges (511, 512); wherein the connection member is movable away from or toward the support member while maintaining a distance between the first engaging member and the second engaging member, and wherein the first guide portion and the second guide portion are each formed of a first long hole and a second long hole respectively such that a distance therebetween in a direction vertical to a moving direction of the connection member becomes greater with movement away from the support member; and wherein in a case where the connection member moves away from the support member, the first arm and the second arm each pivotally move so as to approach each other; in a case where the connection member moves toward the support member, the first arm and the second arm each pivotally move so as to away from each other. a support member; a first arm having an end pivotally supported by the support member; a second arm disposed to face the first arm, having an end pivotally supported by the support member; a first concave portion formed on the first arm on a side facing the second arm and formed in a curved shape corresponding to the circular flanges (511, 512) to fit a first region along the outer peripheries of the circular flanges (511, 512); a second concave portion formed on the second arm on a side facing the first arm and formed in a curved shape corresponding to the circular flanges (511, 512) to fit a second region along the outer peripheries of the circular flanges (511, 512); a connection member connecting the first arm and the second arm; a first guide portion formed close to the end of the first arm and engaged with a first end of the connection member through a first engaging member; and a second guide portion formed close to the end of the second arm and engaged with a second end of the connection member through a second engaging member; a third concave portion formed on the connection member on an opposite side to the support member side, to fit a third region along the outer peripheries of the circular flanges (511, 512); wherein the connection member is movable away from or toward the support member while maintaining a distance between the first engaging member and the second engaging member, and wherein the first guide portion and the second guide portion are each formed of a first long hole and a second long hole respectively such that a distance therebetween in a direction vertical to a moving direction of the connection member becomes greater with movement away from the support member; and wherein in a case where the connection member moves away from the support member, the first arm and the second arm each pivotally move so as to approach each other; in a case where the connection member moves toward the support member, the first arm and the second arm each pivotally move so as to away from each other.