A tensioning device which applies upward tension to a bolt fastened to a fastened member, having a first male thread portion formed on an outer circumferential surface of a head of the bolt, the tensioning device comprising: a connection member having a first female thread portion (21a1) to be threadly engaged with the first male thread portion; a tensioning mechanism which tensions the bolt via the connection member while having the first male thread portion threadly engaged with the first female thread portion (21a1); and a tension bearer which is disposed around an outer circumference of the connection member, and bears a reaction force applied from the fastened member in tensioning performed by the tensioning mechanism, characterized in that: the tensioning mechanism includes a tension rod, a bearing which rotatably supports the tension rod around a vertically extending shaft portion, and a rotation mechanism serving to rotate the tension rod around the shaft portion; a convex portion having a second male thread portion formed on an outer circumferential surface is disposed on one of an upper end of the connection member and a lower end of the tension rod; and a hollow portion having a second female thread portion to be threadly engaged with the second male thread portion on an inner circumferential surface is formed in the other one of the upper end of the connection member and the lower end of the tension rod. a connection member having a first female thread portion (21a1) to be threadly engaged with the first male thread portion; a tensioning mechanism which tensions the bolt via the connection member while having the first male thread portion threadly engaged with the first female thread portion (21a1); and a tension bearer which is disposed around an outer circumference of the connection member, and bears a reaction force applied from the fastened member in tensioning performed by the tensioning mechanism, characterized in that: the tensioning mechanism includes a tension rod, a bearing which rotatably supports the tension rod around a vertically extending shaft portion, and a rotation mechanism serving to rotate the tension rod around the shaft portion; a convex portion having a second male thread portion formed on an outer circumferential surface is disposed on one of an upper end of the connection member and a lower end of the tension rod; and a hollow portion having a second female thread portion to be threadly engaged with the second male thread portion on an inner circumferential surface is formed in the other one of the upper end of the connection member and the lower end of the tension rod. the tensioning mechanism includes a tension rod, a bearing which rotatably supports the tension rod around a vertically extending shaft portion, and a rotation mechanism serving to rotate the tension rod around the shaft portion; a convex portion having a second male thread portion formed on an outer circumferential surface is disposed on one of an upper end of the connection member and a lower end of the tension rod; and a hollow portion having a second female thread portion to be threadly engaged with the second male thread portion on an inner circumferential surface is formed in the other one of the upper end of the connection member and the lower end of the tension rod.