In order to investigate the critical buckling stress of rectangular plates with longitudinal stiffeners under simple bending moments, the author performed two series of simple bending experiments. The first series dealt with the simple bending experiments of a web plate only, and the second dealt with a box girder with different kinds of web plates. The critical buckling stress of rectangular plates calculated by energy method shows good coincidence with the result of the author's experiments, and there is no significant difference between the experimental results of the critical stresses under simple bending of web plates with a longitudinal stiffener and with two longitudinal stiffeners. Furthermore, it has been made clear that the effect of change of stress distribution over a web plate with longitudinal stiffeners under bending is nearly equal to the effect of the same over a web plate without stiffeners.