The motion of particles under shear stress has been numerically simulated using two-dimensional distinct element method. The calculated results well describe the stick-slip phenomena caused by the friction in the particle bed. The results also show that the motions that govern these phenomena are particle rotations which give rise to the slip motion from the static stick state of the particle bed. These motions of particle rotations are transmitted fairly deep inside the particle bed due to particle-particle interaction. Based on these calculated results. we have also obtained the friction loop which is qualitatively in good agreement with the data measured by Nasuno et al. However. the calculated friction coefficient of the particle bed is about 30% of that measured by Nasuno, et al. The main cause seems to be the difference in the clearance between the cover plate particles and the bed particles from the experimental state in the powder bed.