To automatically fasten and loosen the bolt heated by the bolt heater inserted into the center hole of a bolt without requiring operators to monitor and stand-by the bolt.
A bolt heater 9 is inserted into the bolt hole 3 of a bolt 4 for fastening flanges 1 and 2. Torque is given to the upper nut 5 to be threadedly engaged to the bolt 4 by a nut turning device 11 through a drive belt 13. The elongation value of the bolt 4 due to heat obtained by an extensometer 10, the turning angle detection signal of the nut turning device 11, the predetermined heat elongation quantity of the bolt 4 by a condition setting device 18, and the predetermined turning angle by the upper nut 5 are given to a control device 17. The control device 17 controls the power supply 16 of a bolt heater so that the bolt 4 performs predetermined elongation and the nut turning device 11 turns the upper nut 5 at a predetermined angle.