Abstract: Provided is bolt attaching and detaching device for fastening and loosening bolt by heating the bolt by bolt heater inserted into central hole of the bolt by which fastening and loosening of the bolt can be done automatically without need of operator to watch or attend always. Bolt heater (9) is inserted into bolt hole (3) of bolt (4) for fastening flanges (1, 2) together. Upper nut (5) in thread engagement with the bolt (4) is given rotational force by nut rotating device (11) via drive belt (13). Measured value of elongation indicator (10) for measuring thermal elongation of the bolt (4) and rotating angle detecting signal of the nut rotating device (11) as well as predetermined elongation amount of the bolt (4) and predetermined rotating angle of the upper nut (5) from condition setting unit (18 are given to control unit (17). The control unit (17) controls bolt heater power source (16) as well as controls the nut rotating device (11) so that the upper nut (5) rotates in the predetermined angle.