A clamp includes a first member, a second member, and a fastener. The first member has a first connecting portion and a first supporting portion. The second member has a second connecting portion and a second supporting portion. The first connecting portion and the second connecting portion are twisted clockwise or counterclockwise such that a first clamping portion of the first connecting portion and a second clamping portion of the second connecting portion are tilted to the right or to the left with respect to a front-back direction when seen from above. A spiral hanger is mounted on a support wire by tightening the first member and the second member by the fastener in a state where a support wire is passed through between the first supporting portion and the second supporting portion and a spiral hanger is passed between the first clamping portion and the second clamping portion.