PURPOSE: To remarkably improve the efficiency and reliability of a device for automatically tightening a bolt with tension by providing an automatic controller connected to both a displacement sensor and a driving device and capable of controlling tightening of the bolt according to displacement of the head of the bolt.

CONSTITUTION: A bolt 1 is passed through subjects 10, 11 to be tightened and the initial tightening whereby tension is not applied to the bolt 1 is carried out and a displacement detecting sensor 5 is provided which is capable of highly accurately detecting the amount that the head 1a of the bolt is axially displaced on the extension line of the axis of the bolt 1. The amount of axial displacement of the bolt head 1a caused by tightening of either a nut 2a, 2b or the bolt or both of them more than the initial tightening is measured by the displacement detecting sensor 5. Thereafter, a controller 7 determines whether or not the amount of displacement reaches a predetermined value, and when the predetermined amount of displacement is reached tightening is automatically stopped.