To provide a main girder structure of a truss bridge allowing the assembly of members at an assembly factory and capable of shortening construction term and improving machining precision.

This main girder structure of the truss bridge having the three- dimensional truss structure is constituted such that a main girder supporting a floor slab 1 is composed of a pair of top chords 3a, 3b fixed to a lower face of the floor slab 1 at a fixed interval in the direction of width of the floor slab 1 and connected by a top chord horizontal linking member 2, a pair of bottom chords 5a, 5b arranged below a pair of top chords 3a, 3b in such a way that its interval becomes narrower than the interval of a pair of top chords 3a, 3b and connected by a bottom chord horizontal linking member 4, and a plurality of diagonal members 6 connecting the top chords and the bottom chords mutually.