To provide a thread fastening device capable of achieving a highly accurate thread fastening by using a stepping motor, and a control method thereof.

The device includes: a bit 10; a stepping motor 11; a drive circuit 12 for driving the stepping motor 11 by an inputted PWM signal, a position detector 13 for detecting the position or rotation angle in the rotating direction of the stepping motor 11; and a control circuit 14 for regulating the PWM signal so that a rotary speed obtainable from a detected information can become equal to a set rotary speed, determining whether or not a torque of the stepping motor 11 obtained from the current value given by the PWM signal has reached a set torque, and, when it has reached the set torque, inputting a constant PWM signal for generating the torque not exceeding the set torque to the drive circuit 12.