A tightening tool for tightening a fastener is disclosed which comprises a motor and a main shaft that engages with the fastener. A clutch is disposed between the motor and the main shaft so that the clutch rotates the main shaft by transmitting torque from the motor to the main shaft when a load acting on the main shaft is less than a predetermined value. The clutch shuts off torque transmission from the motor to the main shaft when the load acting on the main shaft reaches or exceeds the predetermined value. When in use, the current flowing to the motor and rotation angle of the main shaft of the motor are detected. The detected current and rotation angle are used to determine whether the tightening torque of the fastener is normal.