A tightening device comprising: a motor; a motor drive circuit that 5 drives the motor; a drive shaft that is rotated by the motor, has a socket mounted on a tip thereof, and is configured to tighten a fastening member; a torque detector that detects a tightening torque that acts on the socket; and a control unit that controls the motor drive circuit based on a set torque set in advance and a measured torque measured by the torque detector, wherein the control unit executes a normal tightening mode in a first tightening process of controlling the motor drive circuit to drive the motor at a first initial output, adjusting an output of the motor after the measured torque that is measured by the 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, or a hard tightening mode in the first tightening process of controlling the motor drive circuit to drive the motor at a first initial output that is less than the first initial output of the normal tightening mode, adjusting the output of the motor after the measured torque that is measured by the torque detector has reached said first control start torque, such that the measured torque reaches said 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 then executes a second tightening process of controlling the motor drive circuit to drive the motor at a second initial output that is less than the first initial output of the hard tightening mode, 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. a motor; a motor drive circuit that 5 drives the motor; a drive shaft that is rotated by the motor, has a socket mounted on a tip thereof, and is configured to tighten a fastening member; a torque detector that detects a tightening torque that acts on the socket; and a control unit that controls the motor drive circuit based on a set torque set in advance and a measured torque measured by the torque detector, wherein the control unit executes a normal tightening mode in a first tightening process of controlling the motor drive circuit to drive the motor at a first initial output, adjusting an output of the motor after the measured torque that is measured by the 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, or a hard tightening mode in the first tightening process of controlling the motor drive circuit to drive the motor at a first initial output that is less than the first initial output of the normal tightening mode, adjusting the output of the motor after the measured torque that is measured by the torque detector has reached said first control start torque, such that the measured torque reaches said 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 then executes a second tightening process of controlling the motor drive circuit to drive the motor at a second initial output that is less than the first initial output of the hard tightening mode, 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.