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The inspection of the slack of the steel high-tension bolt is important in the maintenance of various mechanical parts and buildings, etc. There occurs the difference of the magnetic property of the bolt head when the axis force of the fastened bolt is increased. Therefore, the evaluation of the slack of the bolt is possible by detecting the difference of the magnetic characteristics. In this paper, the electromagnetic method for measuring the slack of the steel high-tension bolt using an alternating magnetic field is proposed. The stress and flux density in the bolt head are estimated by 3-D stress analysis using the finite-element method and nonlinear electromagnetic finite element analysis taking account of the electromagnetic property under various compressive stresses. In addition, the experimental verification is also carried out.