The problems treated here are those the square plate with four clamped edges is under a lateral load. The load has firstly constant value on a part of the plate and secondly symmetrical to its center. The deflection of the plate is assummed of the same order as its thickness. The relation between its maximum deflection i. e., the deflection of its center and the load are obtained. The membrane and bending stresses of the plate are also calculated. These results are compared with the experiment in the case of the concentrated load. In conclusion, I wish to thank Prof. Tomotika for his encouragement during this work. This research has been Department of Education, and certain parts of this paper was read at the meeting of the phyisical society of Japan.