In general, a thin plate is employed as a main member of mechanical structure such as vehicle etc. The ultimate load of a thin plate under tensile force is related with the mechanical property, that is tensile strength. However the behavior against compressive load depends on not only the mechanical properties but also the configuration of the structure, because the thin plate occasionally buckles. Hence, in the actual use under the variety of loads, the reinforcement such as a bent portion or additional rib member is usually installed to the thin plate. However structural designers strongly require how to evaluate the effect of the reinforcement and to estimate the effective distance of the reinforcements. In this paper, the critical load on buckling of thin rectangular plate is investigated experimentally and analytically by shearing force under the fixed boundary condition.