To provide an aerial bypass road for solving the problem that a land for constructing a bypass road for alleviating traffic congestion or a detour bypass road cannot be afforded.

A structure having an entrance lane and an exit lane each constructed on a single lane ramp is employed for minimizing the space occupied by a road, and a viaduct structure 4 for constructing an aerial bypass road is employed. A viaduct 4 has a two-story structure 4 composed of upper/lower stories each having double lanes, and an entrance 2/an exit 1 and the multistory viaduct 4 are connected with each other by single lane ramps 3. A double-lane bypass road or a four-lane bypass road is constituted by using the ramps 3 having a single lane width and the lightweight long-span multistory viaduct. The detour bypass road is constructed in the air above the road by employing the two-story viaduct 4 and the single lane ramps 3 for entering/leaving the detour bypass road and the bypass road for alleviating traffic congestion. Thus, the traffic congestion in an urban area is alleviated and traffic convenience is enhanced.