PURPOSE: To check missing of a washer at the time of tightening a screw by detecting existence of the washer by means of comparing a detected value with a set value by way of an actual time required after screw tightening is started until a screw member on the tightening side is seated or an angle of rotation is detected.

CONSTITUTION: A torque sensor 11 is installed on a fixing part to hold a bolt holder 8 free to rotate and a tightening torque detected by the torque sensor 11 is input to a controller 12 in a bolt tightening device 9. Thereafter, a biting point of time (a) and a seated point of time (c) of a bolt 6 are detected by using the tightening torque. Additionally, a rotational angle sensor 13 is installed on the bolt tightening device 9, and an angle of rotation of a bolt 5 and the bolt holder 8 detected by the sensor 13 is input to the controller 12. A set angle of rotation in the case when there is a washer and a set angle of rotation in the case when there is missed a washer are memorized in the controller 12, and an actual angle of rotation of the bolt 5 is integrated by a signal from the rotational angle sensor 13 and it is compared with the set angle of rotation to judge existence of the washer 6.