PURPOSE: To obtain the axial force acting on the bolt with small dispersion, by exciting the primary coil of a magnetic sensor which is contacted on the top surface of a bolt head with two different exciting frequencies, and obtaining the iron loss for each exciting frequency.

CONSTITUTION: The magnetic sensor 4 is contacted with the top surface of the head of the bolt 2 made of a magnetic material, and the iron loss of the bolt 2 is measured. The axial power acting on the bolt 2 is obtained from the measured iron loss. In this case, two different exciting frequencies are used to excite the primary coil 6 of the magnetic sensor 4, and the iron each exciting frequency is measured. From the difference between the two measured iron loss values, the axial force acting on the bolt 2 is obtained. In this way, the axial force of the bolt 2 characterized by small dispersion can be measured.