The invention aims to reduce an error of an axial force detection or the like by tensioning a bolt with a tensioning device threadly engaged with a part of the bolt, which is different from a shaft portion of the bolt. A tensioning device 1 is configured to apply upward tension to a bolt 10 fastened to a fastened member H, and insertedly fitted with a washer 11. A first male thread portion 11a isformed on an outer circumferential surface of the washer 11. The tensioning device includes a connection member 2 having a first female thread portion 21a1 to be threadly engaged with the first male thread portion 11a, a tensioning mechanism 3 for tensioning the connection member 2 in a state where the first male thread portion 11a is threadly engaged with the first female thread portion 21a1, anda tension bearer 4 which is disposed around an outer circumference of the connection member 2, and bears a reaction force applied from the fastened member H in tensioning performed by the tensioningmechanism 3.