Loosening mechanism of bolted joint was examined experimentally and numerically. The loosening tests were carried out using Junker's type loosening machine. In the Junker's tests, transverse repeated displacements were applied to the bolted joint with a hexagon bolt and nut, and then a reduction in the axial bolt force was measured．In the FEM calculations, the loosening behavior of the bolted joint was examined for the model of type loosening machine.In addition, the effects of the flatness and the surface roughness at the bearing surfaces in the clamped part on the reduction in the axial bolt force were examined in the FEM calculations. As the results, the loosening mechanism was elucidated and it was shown that the effect of the nut rotation was important as well as the elastic torsion of the bolt.The amount of the reduction in the axial bolt force was estimated in the FEM calculation taking account of the flatness and the surface roughness. The numerical results were in a fairly good agreement with the measured results. Discussion was made on the loosening mechanism in bolted joints under transverse repeated displacement.