An error of an axial force detection is reduced by tensioning a bolt with a tensioning device engaged with a part of the bolt, different from a shaft portion of the bolt. A tensioning device is configured to apply upward tension to a bolt fastened to a fastened member H, and fitted with a washer. A first male thread portion is formed on an outer circumferential surface of the washer. The tensioning device includes a connection member having a first female thread portion to be engaged with the first male thread portion, a tensioning mechanism for tensioning the connection member in a state where the first male thread portion is engaged with the first female thread portion, 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 H in tensioning performed by the tensioning mechanism.