To minimize congestion which occurs during a construction period by shortening the construction period.

This viaduct making a level crossing part S of a traffic road into a grade separated crossing comprises a main bridge part 5 disposed above the level crossing part; an approach part 6 connected to a lower side road; and a connecting part 7 connecting the main bridge part with the approach part. The lower construction 25 of at least the connecting part 7 is constituted by arranging a plurality of approximately semicircular leg parts 26 each formed by connecting the top parts of precast concrete arch members 27 to each other, in series in the length direction.