Studies on the plastic design of plane structures subjected to in-plane-load have been performed by many reserchers, and the collapsing loads and mechanisms may be obtained easily in most cases. The minimum weight design methods of such structures have been also established. If the plane structures, however, are loaded outward direction of their plane, they may crash in three dimension. To analyze this problem we must use the yielding condition of the sections which sustain simulteneously two bending moments and a twisting moment therefore the analysis becomes difficult. A few studies are reported on the problem, but they contain only numerical or experimental results of sepecially fixed models and the general solutions are not considered. A pair of king posts is one of the representative plane structures which will collapse spatially. The authors studied the plastic design method of the structures that are loaded perpendicular to thier plane, using the linearlized yielding condition for combined bending and twisting moments introduced by Hodge and Sankaranarayanan. The relations between the collapsing loads and mechanisms, the dimensions of posts and the shapes of sections are obtained systematically. Then we considered the optimum design condition from a view point of weight saving and showed a minimum weight design method for king posts.