The development of the inspection technique of detecting the slack of the high tension bolt used in an elevated bridge, a railway viaduct, and various mechanical parts, etc. is desired. The permeability inside the side of the bolt head is changed since the bolt head is compressed by bolting a plate. Therefore, the evaluation of the slack of the bolt is possible by detecting the difference of the magnetic characteristics. In this paper, the electromagnetic inspection method for measuring the slack of the steel high-tension bolt of hexagon head using the small alternating magnetic field probe sensor is proposed. The stress and flux density near the bolt head are estimated by 3-D nonlinear electromagnetic finite element method (FEM) analysis taking account of the electromagnetic property under various compressive stresses. The results showed that the non-destructive inspection of the slack of the high-tension bolts is possible by detecting the change of flux density in the proposed small inspection probe sensor.