An analysis by the matrix method is given to a plate, stiffened in one direction, provided with web frames or girders and ordinary frames and subjected to lateral load. The basic equation for the stiffened plate is introduced by combining the plane stress theory and the bending theory of plate. The normal stress distribution in the mid-surface is generally different at each intersection, that is the peak of stress at the intersection of ordinary frame and plate is higher than that in the position of web frame. And the less stiffness of the ordinary frame is, the higher peak of stress appears at the intersection. The experimental results agree fairly well with theoretical ones. 1. Introduction / 2. Assumptions / 3. Stiffness matrix for the state of plane stress of plate / 4. Example / 6. Conclusion