Abstract: A device and method for variable torque fastening are capable of easily varying a tightening torque and are capable of automatically changing a tightening torque according to the sequence set by an operator. The variable torque fastening device includes a fastening unit having a drive shaft which is rotated by a motor, a driven shaft which is rotated by the drive shaft and is mounted with a fastening tool, a slip clutch which intermittently connects the drive shaft to the driven shaft, and a piston which moves forward and backward by an air pressure to press the slip clutch, a compressed air supply source which supplies compressed air to the fastening unit to press the piston, and an air pressure control unit which controls a pressure of the air that presses the piston.