PURPOSE: To obtain strength and a secondary moment of inertia higher than those of honeycomb structure by a sheet of a plate, by a method wherein processing such as bending. curving of a plate is perfomed by a die roll, corner parts are deformed into acute angles, different positions of the plates are brought into contact directly with each other or indirectly with each other by pinching the plate and a two-dimensional closed space is formed.

CONSTITUTION: A sheet of plate is bent or deep-drawn, a continuous primary molded boby is made and plastic deformation is imparted further to its nook part or corner part 1 respectively by a processing jig 6 such as a roll or a die so that the nook part or the corner part 1 becomes an acute angle. On the one hand, plastic deformation is imparted to neither a top plate 3 nor a bottom plate 4 and bend processing is performed at the larger radius of curvature so as to be limited to elastic deformation. When the processing jig is removed after that, the nook part or the corner part 1 keeps an imparted form of the acute angle given by plastic deformation, the top plate 3 and bottom plate 4 are spling-backed horizontally and becomed a three- dimensional body. The three-dimensional structural body whose strength in a longitudional direction is extremely high and weight is small is obtained by two stage processing through the sheet of the plate like this.