PURPOSE: To implement the method by which the axial force of the bolt is accurately controlled and manufacturing man-hours of the bolt is reduced, by detecting the position where the energy of the ultrasonic wave which is transmitted from the surface of the object to be tightened is changed by a specified amount.

CONSTITUTION: An ultrasonic wave probe 11 is arranged in the vicinity of the bolt on the surface 12 of the objects 3 which are tightened by the bolt 1 and a nut 2. The ultrasonic wave is transmitted in the direction toward a contact interface 8 forming a boundary part of the objects to be tightened from the surface 12. In this case, when the ultrasonic probe 11 is arranged at a point A on the top surface 12 of the object to be tightened 3, the transmitted ultrasonic wave passes through the contact interface 8, reflected by a lower surface 13, and returned. When the ultrasonic wave probe 11 is arranged on a point B, the ultrasonic wave is reflected by the contact interface 8 under the open state, and returned. Based on the correlation of the positions at which the reflected echo changes and the axial force Ff of the bolt 1, the axial force Ff which is generated in said bolt 1 is detected, and its adequacy for practical use is controlled.