Abstract: In order to prevent a looseness of bolts after main clamping, a nut runner of the present invention comprises a torque sensor for detecting a clamping torque T of a clamping unit, and a torque comparison means for comparing the clamping torque T with a clamping completion torque ST. The nut runner further includes a torque control means which gradually reduces a rotational torque of the motor until a resilience such as a torsion accumulated in the clamping unit is entirely nullified when the clamping torque T reaches the clamping completion torque ST. This ensures an effective cancellation of an inverse torque arising from the resilience and acting in the direction loosening the bolt, thus preventing the bolt from loosening.