A tensioning device that is configured to apply upward tension to a bolt-nut fastener including a bolt insertedly fitted with a fastened member, and a nut to be fastened to a shaft of the bolt, the shaft protruding from the fastened member, and the nut having a first male thread portion formed on an outer circumferential surface, the tensioning device comprising: a connection member having a first female thread portion (21a1) that is configured to be threadly engaged with the first male thread portion; a tensioning mechanism that is configured to tension the nut 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 is configured to bear a reaction force applied from the fastened member in tensioning performed by the tensioning mechanism, wherein the tension bearer has an axial force detection unit, characterized in that: the tensioning mechanism includes a tension rod, a bearing and a rotation mechanism, the tension rod comprising a small-diameter rod portion and a large-diameter rod portion, the small-diameter rod portion and the large-diameter rod portion being integrally formed, the bearing rotatably supporting the small-diameter rod portion and being vertically interposed between the large-diameter rod portion and the tension bearer, and the rotation mechanism serving to rotate the tension rod; 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; 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; and the tension bearer is configured to be interposed between the bearing and the fastened member in the vertical direction. a connection member having a first female thread portion (21a1) that is configured to be threadly engaged with the first male thread portion; a tensioning mechanism that is configured to tension the nut 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 is configured to bear a reaction force applied from the fastened member in tensioning performed by the tensioning mechanism, wherein the tension bearer has an axial force detection unit, the tensioning mechanism includes a tension rod, a bearing and a rotation mechanism, the tension rod comprising a small-diameter rod portion and a large-diameter rod portion, the small-diameter rod portion and the large-diameter rod portion being integrally formed, the bearing rotatably supporting the small-diameter rod portion and being vertically interposed between the large-diameter rod portion and the tension bearer, and the rotation mechanism serving to rotate the tension rod; 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; 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; and the tension bearer is configured to be interposed between the bearing and the fastened member in the vertical direction.