A two-dimensional model of granular materials was made by using polyurethane rods, which are utilized as a photoelastic material. Simple shear tests on the model beds were carried out in order to observe microscopic internal mechanisms. The rotation angle (α) of particles, the contact angle (θ) of contact points, interparticle forces (f) and their inclination angles (δ) were detected at a constant interval of shear strain (γ). The interaction between adjacent particles was calculated from the observed results, and the changes of contact direction (Δθ/Δγ), sliding movements (Δψ/Δγ) and rolling movements (Δρ/Δγ) at contact points were obtained. Their statistical values (the average and standard deviation) were investigated. It was found that these values depended on the contact angle as well as on the interparticle force. The interaction between particles became remarkable with increasing shear strain. Therefore, the number of contact points in a special direction increased remarkably during the early stage of shear, and then, it decreased slightly during the steady state of shear. The stress-strain relation was calculated from the microscopic behavior, and was in good agreement with the experimental one.