Abstract: During the period from the start until the completion of tightening of a pipe joint, the tightening torque applied to the pipe joint and the number of turns are sampled at prescribed intervals and stored. After the completion of tightening, a differential calculation is performed with respect to the stored data to determine the rate of change of the tightening torque with respect to the number of turns. Next, a reference value and the previously determined differentials are compared in the reverse order from storing, i.e., going backwards from the value corresponding to the completion of tightening. Based on this comparison, the stored data corresponding to the differential value which first falls below the reference value are made the tightening torque and the number of turns at the transition point, and the tightened condition is assessed based on whether the tightening torque and/or the number of turns at the transition point are in prescribed ranges.