To smooth the passage of an automobile by installing an upper-floor circular road on the upper floor of an elevated structure built while crossing an at-grade intersection, erecting ramp bridges among the inlet and outlet lanes of the intersection near to the center lines of crossed each road and the upper floor of the elevated structure and connecting the ramp bridges to the upper floor circular road.

An elevated structure D is installed while crossing the at-grade intersection C of a trunk road A and a frontage road B. A circular road 7 is set up onto the upper floor 5 of the elevated structure D. Ramp bridges F for highway grade separation are mounted among the upper sections of approach and outlet lanes to the intersection near to the center lines of the crossed trunk road A and frontage road B and the end section of the upper floor 5 of the elevated structure D. Crossing gates 9 are attached to the drive-in ports of each ramp bridge F. Accordingly, the multi-layerization of mutual intersections of multilanes is facilitated, and an automobile can pass through the intersection easily.