To reduce the thickness of a concrete layer and construct the whole light and at a low cost by joining a lower surface plate and an upper surface plate which are made by a steel plate to each other at a designated space through an intermediate support member to form a double structure, and integrally forming concrete with a designated thickness on the upper surface to be synthesized in strength.

A bridge 1 is so constructed that a main structure 10 formed by a main girder 11 and a cross beam 12 is supported on a bridge pier 2 through a support 3, and a floor system 20 is supported on the main structure 10. The main structure 10 is so constructed that a pair of main girders 11 are disposed right and left in the cross direction, and cross beams 12 are disposed at designated spaces between the main girders 11. The main girders 11 and the cross beams 12 are respectively formed by a steel plate to be like an I-shape with a designated height in cross section, the cross beams 12 have side edges fixed to the web of the main girder 11 by welding, and they are supported at an intersecting connecting region of the main girder 11 and the cross beam 12 on a bridge pier 2. The floor system 20 is so constructed that a concrete layer 22 with a designated thickness is formed on a steel plate double structure 21, and the steel plate double structure 21 is fixed on the upper surface of the main girder 11 by a bolt and a nut.