PURPOSE: To aim at smooth insertion of a nut and as well at preventing a nut from being caughted up and damaged, by providing a torque change-over device for establishing a predetermined torque after operation under a lower torque upon nut insertion and a rotational direction change over device which reversely rotates a drive source when the drive source does not work due to excessive torque before operation.

CONSTITUTION: When an abutment 28 touches a microswitch 29 after starting of fastening a nut so that the microswitch 29 is turned on, a torque change-over device 42 is actuated to switch over the torque set to a drive source 1 into the final torque, and the fastening is made while a compression spring acts upon the nut. When the abutment 31 abuts against a lever 32, the lever 32 separates from a switch 34 which is therefore turned off, and the fastening level of the nut is detected. At this time if the threaded part of the nut is caughted up since the insertion of the nut is not good so that the drive source 1 does not work due to excessive torque, the drive source 1 is reversely rotated by means of a rotational direction change-over switch 43 to change over the set torque into the larger one so that the nut is removed. After a predetermined time period the drive source is again normally rotated to change over the set torque into the smaller one for lasting the fastening of the nut. Accordingly, smooth fastening of nuts may be always made.