To provide a steel/concrete synthetic girder structure, of which the cost is low and the construction is easy.

The steel/concrete synthetic girder structure 10 comprises a lower floor slab 11 of a concrete structure, an upper floor slab 14 of a concrete structure, and trunk members 12, 13 arranged between the lower floor slab 11 and the upper floor slab 14 and coupling both floor slabs to constitute the trunk of the girder structure 10. The trunk members 12, 13 comprises steel arrow plates 15 having fitting couplings 15d and the like on both ends, the couplings being fitted into each other to make joints, where a plurality of the steel arrow plates 15 are joined along the longitudinal directions of the lower floor slab 11 and the upper floor slab 14.