A tightening device is able to check that a fastening member has been fastened to a desired set torque or greater after being tightened, without loosening the fastening member. The tightening device is provided with a mode switching switch for switching between a first tightening process of adjusting the output of a motor after measured torque that is measured by a torque detector has reached a preset first control start torque, such that the measured torque reaches a preset first set torque in a phased manner through repeated increases and decreases in torque, and blocking power supply to the motor when the measured torque reaches the first set torque, and a second tightening process of driving the motor at an initial output that is less than in the first tightening process, adjusting the output of the motor after the measured torque that is measured by the torque detector has reached a preset second control start torque, such that the measured torque reaches a preset second set torque in a phased manner through repeated increases and decreases in torque, and blocking power supply to the motor when the measured torque reaches the second set torque.