PURPOSE: To improve work efficiency by fitting a main girder fit recessed part provided on the back of a panel-like steel floor board to a main girder from above, and joining an upper flange of the main girder to a deck plate of the steel floor board by high strength bolts.

CONSTITUTION: Each main girder fit recessed part 2 of a steel floor board 1 is placed on an upper flange 4 of a main girder 3, and a deck plate 5 of the floor board 1 and the flange 4 are joined to each other by many high strength bolts 6. Subsequently, joint plates 7 of floor boards adjacent to each other in the longitudinal direction of the main girder 3 are joined to each other by many high strength bolts. Subsequently, the end parts of the joint plates 7 and lateral ribs and a fitting plate fixed to a web of the main girder 3 are joined to each other by many high strength bolts. Thus, construction can be executed quickly and easily without dry worker having skill, and as a splice plate is not used, influence upon a paving material can be reduced. Furthermore, the respective parts can be divided to be easily carried and bound more firmly.